e.g., methyl, ethyl, propyl, butyl and pentyl groups, ##STR3## in which either one of R7 and R8 is a hydroxy group, the other being a hydrogen atom, and ##STR4## in which R9 is a divalent group selected from the class consisting of 4-hydroxybenzylidene, 2-hydroxyphenylimino and 2-hydroxy ??-naphthylimino groups.