  Bachetti et al. (2021) have recently claimed to measure the mass transfer
rate in the pulsing ULX system M82 X-2 by following the change of its orbital
period over 7 yr. We reiterate the known point that this method cannot give a
reliable result (or even necessarily predict the correct sign of long-term
period change) without a far longer baseline (here $\gg 1000$~yr) or for
systems with a much higher long-term mass transfer rate ($\gg 10^{-4} \rm
M_{\odot}/{\rm yr}$), if they exist. Applying the method of Bachetti et al.
(2021) to measured orbital period derivatives predicts that the well-studied
quiescent X-ray transients XTEJ1118+480, A0620-00 should currently instead have
steady accretion discs and be bright X-ray sources, while Nova Muscae 1991
should be still brighter (a ULX). But all three sources are observed to be
extremely faint. We conclude that there is no evidence to support the high mass
transfer rate that Bachetti et al. (2021) find for M82 X-2 as it is deduced
from period noise not related to the binary evolution.
