The system parameters of the polars MR Ser and ST LMi

We obtain the NaI 8183,8195 absorption line radial velocity curves for thepolars ST LMi and MR Ser, from which we find the semi-amplitudes to beK_abs=329=+/-6 kms-1 and K_abs=289+/-9 kms-1 respectively. We find that forboth systems the effects on the \NaI absorption lines due to X-rays heating theinner face of the secondary are negligible, and so the values obtained forK_abs can be taken as the true semi-amplitude of the secondary star. We thendetermine the projected rotational velocities, vsini, to be 104+/-9 kms-1 and66+/-13 kms-1 for ST LMi and MR Ser respectively which enables their massratios to be calculated. For ST LMi and MR Ser we find the mass ratio to be0.22+/-0.04 and 0.10+/-0.05 respectively; values which are significantlydifferent only at the 94 percent level. We show that ``spike'' in the orbitalperiod distribution of polars is a significant feature, although the discoveryof only one more system with a period outside the ``spike'' would decrease itssignificance below a 99 percent confidence level. We conclude that, even if thelimb darkening coefficients for the secondary stars in ST LMi and MR Ser arethe 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.Comment: 6 pages, accepted for MNRAS, use mn.sty, 9 postscript figures, 3 table