Question: Was it author Dean Koontz or author Kurt Vonnegut who used pen names early in their career?

Context articles: Bernard Vonnegut I
Whispers (novel)
A Man Without a Country
Dean Koontz
Kurt Vonnegut
Kurt Vonnegut Sr.
City of Night (Koontz and Gorman novel)
Clemens Vonnegut
Prodigal Son (novel)
Wandering Stars

Bernard Vonnegut I, WAA, FAIA, (August 8, 1855 – August 7, 1908) was an American lecturer and architect active in late-nineteenth and early-twentieth-century Indiana.
 He was a co-founder of the locally renowned Indianapolis architectural firm of Vonnegut and Bohn, and was active in a range of residential, religious, institutional, civic, and commercial commissions.
 He is the namesake and grandfather of scientist Bernard Vonnegut, father of the architect Kurt Vonnegut Sr., and grandfather of author Kurt Vonnegut.
Whispers is a novel by American suspense author Dean Koontz, originally published in 1980.
 It was the first of Koontz's novels to appear on the "New York Times" Best Seller list, and is widely credited with launching his career as a best-selling author.
 The novel was also adapted for a 1990 film by the same name.
A Man Without a Country (subtitle: "A Memoir Of Life In George W Bush's America") is an essay collection published in 2005 by the author Kurt Vonnegut.
 The extremely short essays that make up this book deal with topics ranging from the importance of humor, to problems with modern technology, to Vonnegut's opinions on the differences between men and women.
 Most prevalent in the text, however, are those essays that elucidate Vonnegut's opinions on politics, and the issues in modern American society, often from a decidedly humanistic perspective.
 In January 2007, Vonnegut indicated that he intended this to be his final work, a statement that proved to be correct with his death in April 2007.
 Later published works of Vonnegut's were all published posthumously, and consisted almost entirely of previously unpublished material from early in his career.
Dean Ray Koontz (born July 9, 1945) is an American author.
 His novels are broadly described as suspense thrillers, but also frequently incorporate elements of horror, fantasy, science fiction, mystery, and satire.
 Many of his books have appeared on "The New York Times" Bestseller List, with 14 hardcovers and 14 paperbacks reaching the number-one position.
 Koontz wrote under a number of pen names earlier in his career, including "David Axton", "Leigh Nichols", and "Brian Coffey".
Kurt Vonnegut Jr. ( ; November 11, 1922April 11, 2007) was an American writer.
 In a career spanning over 50 years, Vonnegut published 14 novels, three short story collections, five plays, and five works of non-fiction.
 He is most famous for his darkly satirical, best-selling novel "Slaughterhouse-Five" (1969).
Kurt Vonnegut Sr. (November 24, 1884 – October 1, 1957) was an American architect and architectural lecturer active in early- to mid-twentieth-century Indianapolis, Indiana.
 A member of the American Institute of Architects, he was partner in the firms of Vonnegut & Bohn, Vonnegut, Bohn & Mueller, and Vonnegut, Wright, and Yeager.
 He designed several churches, banks, and became the inhouse architect for Indiana Bell and Hooks Drug stores (prior to World War II), practicing extensively in the Art Deco style.
 He was the father of chemist Bernard Vonnegut and author Kurt Vonnegut Jr.
City of Night is a novel released in 2005 by the best-selling author Dean Koontz and Ed Gorman.
 The book is the second in Koontz's series, entitled "Dean Koontz's Frankenstein".
 The third book in the series, "Dead and Alive", was published in 2009.
Clemens Vonnegut Sr. (November 20, 1824 – December 13, 1906) was a German emigrant to the United States and successful businessman.
 He was the patriarch of the prominent German-American Vonnegut clan (later Schnull-Vonnegut) of Indiana – he was the father and grandfather of architects Bernard Vonnegut I and Kurt Vonnegut Sr., respectively, and great-grandfather of scientist Bernard Vonnegut and author Kurt Vonnegut Jr.
Prodigal Son is a novel by the best-selling author Dean Koontz, released in 2005.
 The book is the first book released by Koontz in a series of five, entitled "Dean Koontz's Frankenstein".
 The book was co-authored by Kevin J. Anderson.
Wandering Stars is an anthology of Jewish fantasy and science fiction, edited by American writer Jack Dann, originally published by Harper & Row in 1974.
 It represented, according to the book cover, "the first time in science fiction that the Jew - and the richness of his themes and particular points of view -- will appear without a mask."
 In his introduction, "Why Me?"
, Isaac Asimov discussed how many Jewish science fiction writers prior to that time had used gentile pen names in order to get published: "Many of the Jewish pulp writers, however, used pen names as a matter of sound business.
 A story entitled "War Gods of the Oyster-Men of Deneb" did not carry conviction if it was written by someone named Chaim Itzkowitz."
 He then goes on to discuss the pen names of various Jewish writers included in this book.
 "Wandering Stars" is therefore of historical significance as the first science fiction anthology where Jewish writers openly identified themselves as such.
 It was followed by a second anthology, "More Wandering Stars", also edited by Jack Dann, published by Doubleday in 1981.