Rotational velocities of the giants in symbiotic stars - I. D'-type symbiotics*

We have measured the rotational velocities (v sini) of the mass donors in a number of D'-type symbiotic stars (SSs), using the cross-correlation function method. Four out of five D' SSs with known v sini appeared to be very fast rotators compared with the catalogues of v sini for the corresponding spectral types. At least three of these stars rotate at a substantial fraction (>~0.5) of the critical velocity. This means that at least in D'-type SSs the cool components rotate faster than isolated giants. If these binary stars are synchronized, their orbital periods should be relatively short (4-60 d). We also briefly discuss the possible origin of the rapid rotation and its connection with mass loss and dust formation.