Springfield was born Mary Isobel Catherine Bernadette O'Brien on 16 April 1939 in West Hampstead, the second child of Gerard Anthony "OB" O'Brien (1904-1979) and Catherine Anne "Kay" O'Brien (nee Ryle; 1900-1974). Her older brother, Dionysius P. A. O'Brien (born 2 July 1934), was later known as Tom Springfield. Springfield's father, who had been raised in British India, worked as a tax accountant and consultant. Her mother came from an Irish family, originally from Tralee, County Kerry, that included a number of journalists.

After finishing school, Springfield sang with Tom in local folk clubs. In 1957 the pair worked together at holiday camps. The following year Springfield responded to an advertisement in The Stage to join The Lana Sisters, an "established sister act", with Iris 'Riss' Long (aka Riss Lana, Riss Chantelle) and Lynne Abrams (not actually sisters). She had changed her name to Shan, and "cut her hair, lost the glasses, experimented with makeup, fashion" to become one of the 'sisters'. As a member of the pop vocal trio, Springfield developed skills in harmonising and microphone technique and recorded, performed on TV, and played at live shows in the United Kingdom and at United States Air Force bases in continental Europe. In 1960, Springfield left The Lana Sisters and formed a pop-folk trio, The Springfields, with Tom and Reshad Feild (both ex-The Kensington Squares), who was replaced by Mike Hurst in 1962. The trio chose their name while rehearsing in a field in Somerset in the springtime and took the stage names of Dusty, Tom, and Tim Springfield. Intending to make an authentic US album, the group travelled to Nashville, Tennessee, to record Folk Songs from the Hills. The local music that Springfield heard during this visit, in particular "Tell Him," helped turn her style from folk and country towards pop music rooted in rhythm and blues. The band was voted the "Top British Vocal Group" by the New Musical Express poll in 1961 and 1962. During early 1963, The Springfields recorded their last UK Top 5 hit, "Say I Won't Be There". The group appeared on ITV Associated Rediffusion's popular music TV series Ready Steady Go!. Springfield left the band after their final concert in October 1963. After the Springfields disbanded, Tom continued songwriting and producing for other artists, including Australian folk-pop group The Seekers, mid-1960s hits "I'll Never Find Another You" and "The Carnival is Over" (lyrics only), and he co-wrote their "Georgy Girl". He also wrote additional tracks for Springfield and released his own solo material.

Using a quote from the above article, answer the following question: What was the ad?
to join The Lana Sisters, an "established sister act",