Background: Alfred Cellier (1 December 1844 - 28 December 1891) was an English composer, orchestrator and conductor. In addition to conducting and music directing the original productions of several of the most famous Gilbert and Sullivan works and writing the overtures to some of them, Cellier conducted at many theatres in London, New York and on tour in Britain, America and Australia. He composed over a dozen operas and other works for the theatre, as well as for orchestra, but his 1886 comic opera, Dorothy, was by far his most successful work. It became the longest-running piece of musical theatre in the nineteenth century.
Context: In December 1877 Cellier joined the D'Oyly Carte company as musical director at the Opera Comique in London. There he conducted The Sorcerer (1877), H.M.S. Pinafore (1878, for which he wrote the overture, based on themes from the opera), Trial by Jury (1878), George Grossmith's Cups and Saucers (1878-79), and three of his own one-act works: Dora's Dream (1877-78 revival), The Spectre Knight (1878), and After All! (1878-79). Cellier was conducting the performance of Pinafore during which the partners of The Comedy Opera Company attempted to repossess the set, and he was noted for his attempts to calm the audience during the fracas. His brother, Francois, succeeded him as musical director at the Opera Comique in 1879. Alfred Cellier was a conductor of a series of promenade concerts at the Queen's Theatre, Long Acre and, in 1878-1879 he was joint conductor, with Sullivan, of the Covent Garden Promenade Concerts.  In 1879, he travelled with Gilbert, Sullivan, and Carte to America, where he acted as conductor for Pinafore and The Pirates of Penzance, with Carte's first American touring company. Cellier prepared the overture to Pirates using Sullivan's music from the rest of the score. Back in London in July 1880, he directed the music at the Opera Comique for Pirates and another of his own pieces with Desprez, In the Sulks. In April 1881, he left the D'Oyly Carte company, ceding the baton to his brother. Cellier composed a three-act grand opera, Pandora, a version of Longfellow's The Masque of Pandora (with a libretto by B. C. Stephenson) that was presented in Boston in 1881. He returned to America later that year as music director of D'Oyly Carte's New York and touring productions of Billee Taylor (1882), Les Manteaux Noirs and Rip Van Winkle (both in the fall of 1882), and Iolanthe (1882-83), for the latter of which he prepared the New York overture.  In 1883, Cellier's setting of Gray's Elegy, in the form of a cantata, was produced at the Leeds music festival. In 1883, Cellier left the D'Oyly Carte company, but he was back for brief periods as music director with D'Oyly Carte's touring companies for Princess Ida (1884) and The Mikado (1885). In 1885, also, Cellier composed incidental music for a production of As You Like It. He composed two more companion pieces that had Savoy Theatre premieres: The Carp (performed with The Mikado and Ruddigore in 1886-87), and Mrs. Jarramie's Genie (composed together with his brother Francois, with libretti by Desprez, which played together with several different operas at the Savoy between 1887 and 1889).  While in London, Cellier conducted at several London theatres, including the Criterion, the St James's, and the Savoy. Later, as a result of ill health, he resided abroad, notably in America and Australia, where he was a representative for the D'Oyly Carte company.
Question: what is D'Oyly Carte?

Answer:
In December 1877 Cellier joined the D'Oyly Carte company as musical director at the Opera Comique in London.