NICER observation list: Target ID 261101

We propose observing CAL 83 and MR Vel, two supersoft X-ray binaries, one in the Large Magellanic Cloud and one in the Galaxy. We will measure possible spectral changes over a period of many years since the last X-ray observations, and periodic modulations with time scales of tens of seconds or tens of minutes. The high luminosity and the not-exceedingly-supersoft spectrum of these two targets makes them ideal candidates for NICER, both from the point of view of feasibility and scientific interest and return. We will monitor the evolution and the amplitude changes of a 67 s period of CAL 83, search for short periodicities in the light curve of MR Vel, and measure again other, longer periods, of 38 minutes and 1 hour respectively, observed several years ago in each the two sources.