Spectroscopy of the white-dwarf companions of PSR~0655+64 and 0820+02

We present spectra of the white-dwarf companions of the radio pulsars 0655+64 and 0820+02. For the latter, we find a spectrum showing strong lines of hydrogen, i.e., that of a DA star. From modelling these lines, the mass of a white dwarf can, in principle, be determined accurately, thus leading to constraints on the evolution of the binary and the mass of the neutron star. Our present spectrum is not of sufficient quality to set a strong limit, but it does indicate that the white dwarf most likely has a low mass. This is consistent with the star being a helium white dwarf, as would be expected from considerations of the mass function and the preceding evolution. From similar considerations, the companion of PSR 0655+64 is expected to be a more massive, carbon-oxygen white dwarf. This is confirmed by our spectra, which show the Swan bands of molecular carbon, making it a DQ star. Unlike what is observed in other DQ stars, the strength of the Swan C2 bands changes drastically, by a factor two in about two hours. We suggests this reflects large-scale surface inhomogeneities which are rotated in and out of the observed hemisphere. If so, this would imply that the white dwarf rotates supersynchronously.