A dyestuff of the formula ##STR1## in which R represents alkyl, alkenyl, cycloalkyl, aralkyl, aryl or, preferably, hydrogen, R.sub.1 represents alkyl, alkenyl, cycloalkyl, aralkyl, a heterocyclic structure or, preferably, aryl, X represents --COOH, --COO.sup..crclbar. cation.sup..sym., a modified carboxyl group, the radical --SO.sub.2 --T or a heterocyclic radical, T being alkyl, alkenyl or aryl, and M represents hydrogen or a cation and A and B can carry non-ionic substituents and/or a --COOH group. The new dyestuffs dye polyester fibers in fast yellow and orange shades.
