In a data sample of approximately 1.3 fb(-1) collected with the D0 detector between 2002 and 2006, the orbitally excited charm state D-s1( /-)(2536) has been observed with a measured mass of 2535.7 /- 0.6(stat) /- 0.5(syst) MeV/c(2) via the decay mode B-s(0) -} D-s1(-)(2536)mu( )nu X-mu. A first measurement is made of the branching ratio product Br((b) over bar -} D-s1(-)(2536)mu( )nu X-mu) x Br(D-s1(-) -} D*K--(S)0).
Assuming that D-s1(-)(2536) production in semileptonic decay is entirely from B-s(0), an extraction of the semileptonic branching ratio Br(B-s(0) -} D-s1(-)(2536)mu( )nu X-mu) is made.