Given the following context:  In the autumn of 1927, Messiaen joined Dupré's organ course. Dupré later wrote that Messiaen, having never seen an organ console, sat quietly for an hour while Dupré explained and demonstrated the instrument, and then came back a week later to play Johann Sebastian Bach's Fantasia in C minor to an impressive standard. From 1929, Messiaen regularly deputised at the Église de la Sainte-Trinité, Paris, for the organist Charles Quef, who was ill at the time. The post became vacant in 1931 when Quef died, and Dupré, Charles Tournemire and Widor among others supported Messiaen's candidacy. His formal application included a letter of recommendation from Widor. The appointment was confirmed in 1931, and he remained the organist at the church for more than sixty years. He also assumed a post at the Schola Cantorum de Paris in the early 1930s. In 1932, he composed the Apparition de l'église éternelle for organ. He married the violinist and composer Claire Delbos also in 1932. Their marriage inspired him to both compose works for her to play (Thème et variations for violin and piano in the year they were married) and to write pieces to celebrate their domestic happiness, including the song cycle Poèmes pour Mi in 1936, which he orchestrated in 1937. Mi was Messiaen's affectionate nickname for his wife. In 1937 their son Pascal was born. The marriage turned to tragedy when Delbos lost her memory after an operation towards the end of World War II. She spent the rest of her life in mental institutions.In 1936, along with André Jolivet, Daniel-Lesur and Yves Baudrier, Messiaen formed the group La jeune France ("Young France"). Their manifesto implicitly attacked the frivolity predominant in contemporary Parisian music and rejected Jean Cocteau's 1918 Le coq et l'arlequin in favour of a "living music, having the impetus of sincerity, generosity and artistic conscientiousness". Messiaen's career soon departed from this polemical phase.  answer the following question:  Whose nickname was Mi?
A:
Claire Delbos