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Summary We report the first soft X-ray light curves of the AM Her system MR Ser, all obtained within a 4-month interval in 1984. The source was serendipitously detected on three occasions by the EXOSA T low-energy telescope, and one observation was made in which MR Ser was the target. MR Ser was probably in a high optical state throughout, although it was seen at half the previous intensity in the final observation. The folded light curves change remarkably between the different observations. In April the soft X-ray light curve showed a minimum around linear polarization phase ϕ = 0.0, and a less pronounced short dip close to phase ϕ ˜ 0.3. In the other observations a broad minimum in the phase range ϕ = 0.3–0.5 is seen. We attribute all the features around phase ϕ ˜ 0.3 to the accretion stream. Some of the changes in shape of the soft X-ray light curve are similar to those of QQ Vul, V834 Cen and possibly AM Her itself. From the observation in which MR Ser was the target, we derive an upper limit to the soft X-ray temperature of kTbb &amp;lt; 120 eV, noting, however, that surface flux limits suggests kTbb &amp;lt; 53 eV. The source was not seen in the medium energy detector. The hard and soft X-ray data do not allow us to comment on the possibility of a ‘soft X-ray problem’ in this system.

The changing soft X-ray light curve of the AM Her system MR Serpentis (PG 1550 + 191)