We obtain the NaI lambdalambda8183, 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 and 289+/-9 km s^-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 than determine the projected rotational velocities, V_rot sini, to be 104+/-9 and 66+/-13 km s^-1 for ST LMi and MR Ser respectively, and this enables their mass ratios to be calculated. For ST LMi and MR Ser we find the mass ratios to be 0.22+/-0.04 and 0.10+/-0.05 respectively, values which are significantly different only at the 94 per cent level. Using the value for the orbital inclination derived from polarimetric measurements, we determine the mass, and the orbital and rotational velocities of the secondary stars. These are significantly different at less than the 90 per cent level. However, if the limb darkening is the same in both objects, these quantities are significantly different at the 96 per cent level. We present Doppler maps of the NaI absorption and CaII emission in ST LMi and MR Ser. In both systems the NaI absorption covers the secondary star. In ST LMi the Doppler image of the CaII emission shows that it originates at the inner face of the secondary star. In MR Ser, however, the emission lies close to but not on the secondary star. We show that the `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 per cent 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 possibility of the two systems having identical parameters. Our observations are therefore compatible with the theory explaining the `spike' in the period distribution of the AM Hers.