Question: When was it revealed that Van Gogh painted Sunflowers?
About the article "Sunflowers (Van Gogh series)", section "The Paris "Sunflowers"".

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Little is known of Van Gogh's activities during the two years he lived with his brother Theo in Paris, 1886-1888.
The fact that he had painted Sunflowers already is only revealed in spring 1889, when Gauguin claimed one of the Arles versions in exchange for studies he had left behind after leaving Arles for Paris.
Van Gogh was upset and replied that Gauguin had absolutely no right to make this request: "I am definitely keeping my sunflowers in question.
He has two of them already, let that hold him.
And if he is not satisfied with the exchange he has made with me, he can take back his little Martinique canvas, and his self-portrait sent me from Brittany, at the same time giving me back both my portrait and the two sunflower canvases which he has taken to Paris.
So if he ever broaches this subject again, I've told you just how matters stand.
"The two Sunflowers in question show two buttons each; one of them was preceded by a small study, and a fourth large canvas combines both compositions.
These were Van Gogh's first paintings with ""nothing but sunflowers""—yet, he had already included sunflowers in still life and landscape earlier.
The Painter of Sunflowers (in French: Le Peintre de Tournesols) is a portrait of Vincent van Gogh by Paul Gauguin in December 1888.
The portrait was painted when Gauguin visited in Arles.
Van Gogh had asked him to come to Arles, because he wanted to start an art-colony there.
Gauguin however only stayed for two months, because the painters argued. 
When Gauguin agreed to visit Arles, Van Gogh hoped for friendship and for his utopian idea of a collective of artists.
In anticipation that August he painted sunflowers.
When Boch visited again, Van Gogh painted a portrait of him, as well as the study "The Poet Against a Starry Sky."
Boch's sister Anna (1848–1936), also an artist, purchased "The Red Vineyard" in 1890.
In preparation for Gauguin's visit he bought two beds, on advice from his friend the station's postal supervisor Joseph Roulin, whose portrait he painted, and on 17 September spent the first night in the still sparsely furnished Yellow House.
When Gauguin consented to work and live side-by-side in Arles with Van Gogh, he started to work on "The Décoration for the Yellow House," probably the most ambitious effort he ever undertook.
Van Gogh did two chair paintings: "Van Gogh's Chair" and "Gauguin's Chair.
"After repeated requests, Gauguin finally arrived in Arles on 23 October.
During November, the two painted together.
Gauguin painted Van Gogh's portrait "The Painter of Sunflowers: Portrait of Vincent van Gogh," and uncharacteristically, Van Gogh painted some pictures from memory – deferring to Gauguin's ideas in this – as well as his "The Red Vineyard".
Notable amongst these "imaginative" paintings is " Memory of the Garden at Etten".
Their first joint outdoor painting exercise produced "Les Alyscamps", and was conducted at the Alyscamps.
The two visited Montpellier that December and viewed works in the Alfred Bruyas collection by Courbet and Delacroix in the Musée Fabre, but their relationship began to deteriorate.
Van Gogh greatly admired Gauguin, and desperately wanted to be treated as his equal.
But Gauguin was arrogant and domineering, a fact that often frustrated Van Gogh.
They quarreled fiercely about art; Van Gogh felt an increasing fear that Gauguin was going to desert him, as a situation he described as one of "excessive tension" reached crisis point.
The precise chain of events that led to the celebrated incident of van Gogh slicing off his ear is not known reliably in detail.
The only account attesting a supposed earlier razor attack on Gauguin comes from Gauguin himself some fifteen years later and biographers agree this account must be considered unreliable and self-serving.
It does seem likely, however, that by 23 December 1888 van Gogh had realized that Gauguin was proposing to leave and that there had been some kind of contretemps between the two.
That evening van Gogh severed his left ear (wholly or in part, accounts differ) with a razor, inducing a severe haemorrhage.
He bandaged his wound and then wrapped the ear in paper and delivered the package to a brothel frequented by both him and Gauguin before returning home and collapsing.
He was found unconscious the next day by the police and taken to hospital.
The local newspaper reported that van Gogh had given the ear to a prostitute with an instruction to guard it carefully.
In Gauguin's later account he implies that in fact van Gogh had left it with the doorman as a memento for Gauguin.
Van Gogh himself had no recollection of these events and it is plain that he had suffered an acute psychotic episode.
Family letters of the time make it clear that the event had not been unexpected.
He had suffered a nervous collapse in Antwerp some three years before and as early as 1880 his father had proposed committing him to an asylum (at Gheel).
The hospital diagnosis was "generalized delirium", and within a few days van Gogh was sectioned.
During the initial few days of his treatment van Gogh repeatedly asked for Gauguin, but Gauguin stayed away.
Gauguin told one of the policeman attending the case, "Be kind enough, Monsieur, to awaken this man with great care, and if he asks for me tell him I have left for Paris; the sight of me might prove fatal for him."
Gauguin wrote of Van Gogh, "His state is worse, he wants to sleep with the patients, chase the nurses, and washes himself in the coal bucket.
That is to say, he continues the biblical mortifications."
Theo – notified by Gauguin – visited, as did both Madame Ginoux and Roulin.
Gauguin left Arles and never saw Van Gogh again.
Despite the gloomy initial disgnosis, van Gogh made a surprisingly speedy recovery.
He returned to the Yellow House by the beginning of January, but was to spend the following month between the hospital and home, suffering from hallucinations and delusions that he was being poisoned.
In March, the police closed his house after a petition by 30 townspeople (including the Ginoux family), who called him "fou roux" ("the redheaded madman").
Paul Signac visited him in the hospital and Van Gogh was allowed home in his company.
In April, he moved into rooms owned by his hospital physician Dr. Rey after floods damaged paintings in his own home.
Around this time, he wrote, "Sometimes moods of indescribable anguish, sometimes moments when the veil of time and fatality of circumstances seemed to be torn apart for an instant."
Two months later he left Arles and entered an asylum (at his own request) in Saint-Rémy-de-Provence. 
Sunflowers (original title, in French: Tournesols) are the subject of two series of still life paintings by the Dutch painter Vincent van Gogh.
The earlier series executed in Paris in 1887 depicts the flowers lying on the ground, while the second set executed a year later in Arles shows bouquets of sunflowers in a vase.
In the artist's mind both sets were linked by the name of his friend Paul Gauguin, who acquired two of the Paris versions.
About eight months later Van Gogh hoped to welcome and to impress Gauguin again with "Sunflowers", now part of the painted "décoration" that he prepared for the guestroom of his "Yellow House," where Gauguin was supposed to stay in Arles.
After Gauguin's departure, Van Gogh imagined the two major versions as wings of the "Berceuse Triptych", and finally he included them in his exhibit at Les XX in Bruxelles. 
Van Gogh painted several versions of landscapes with flowers, including his"View of Arles with Irises," and paintings of flowers, including "Irises", "Sunflowers", lilacs and roses.
Some reflect his interests in the language of color, and also in Japanese ukiyo-e woodblock prints.
He completed two series of sunflowers.
The first dated from his 1887 stay in Paris, the second during his visit to Arles the following year.
The Paris series shows living flowers in the ground, in the second, they are dying in vases.
The 1888 paintings were created during a rare period of optimism for the artist.
He intended them to decorate a bedroom where Gauguin was supposed to stay in Arles that August, when the two would create the community of artists Van Gogh had long hoped for.
The flowers are rendered with thick brushstrokes (impasto) and heavy layers of paint.
In an August 1888 letter to Theo, he wrote, "I am hard at it, painting with the enthusiasm of a Marseillais eating bouillabaisse, which won't surprise you when you know that what I'm at is the painting of some sunflowers.
If I carry out this idea there will be a dozen panels.
So the whole thing will be a symphony in blue and yellow.
I am working at it every morning from sunrise on, for the flowers fade so quickly.
I am now on the fourth picture of sunflowers.
This fourth one is a bunch of 14 flowers ... it gives a singular effect."