The system parameters of the polars MR Ser and ST LMi

We obtain the NaI 8183,8195 absorption line radial velocity curves for the polars ST LMi and MR Ser, from which we find the semi-amplitudes to be K_abs=329=+/-6 kms-1 and K_abs=289+/-9 kms-1 respectively. We find that for both systems the effects on the \NaI absorption lines due to X-rays heating the inner face of the secondary are negligible, and so the values obtained for K_abs can be taken as the true semi-amplitude of the secondary star. We then determine the projected rotational velocities, vsini, to be 104+/-9 kms-1 and 66+/-13 kms-1 for ST LMi and MR Ser respectively which enables their mass ratios to be calculated. For ST LMi and MR Ser we find the mass ratio to be 0.22+/-0.04 and 0.10+/-0.05 respectively; values which are significantly different only at the 94 percent level. We show that spike'' in the orbital period distribution of polars is a significant feature, although the discovery of only one more system with a period outside the spike’’ would decrease its significance below a 99 percent confidence level. We conclude that, even if the limb darkening coefficients for the secondary stars in ST LMi and MR Ser are the same, we cannot rule out the two systems having identical parameters. Therefore our observations are compatible with the theory explaining the ``spike’’ in the period distribution of the AM Hers.