Spin-resolved optical CCD spectroscopy of GK Persei.

We present the results of spin phase-resolved optical spectroscopy of the intermediate polar GK Persei. Our data show that, in common with other such systems, the spectrum is modulated at the spin period of the white dwarf, but the characteristics of the modulation in GK Per are not typical of the class. The V/R ratio spin modulation shows a `glitch' in phase in the space of one day, whilst an equivalent width modulation shows no such phenomenon. This suggests either that the accretion geometry in GK Per is unstable, or that the modulation pattern is produced by the interplay of multiple spinmodulated components. The fact that H is single-peaked whereas the other Balmer lines are not indicates the presence of an additional emission region producing optically thin Balmer lines via radiative recombination. Key words: binaries: spectroscopic - stars: individual: GK Per - stars: magnetic fields - novae, cataclysmic variables - stars: variables: other - X-rays: stars.