  We study the effect of short term variations of the evolution of AM CVn
systems on their gravitational wave emissions and in particular LISA
observations. We model the systems according to their equilibrium mass-transfer
evolution as driven by gravitational wave emission and tidal interaction, and
determine their reaction to a sudden perturbation of the system. This is
inspired by the suggestion to explain the orbital period evolution of the
ultra-compact binary systems V407 Vul and RX-J0806+1527 by non-equilibrium mass
transfer. The characteristics of the emitted gravitational wave signal are
deduced from a Taylor expansion of a Newtonian quadrupolar emission model, and
the changes in signal structure as visible to the LISA mission are determined.
We show that short term variations can significantly change the higher order
terms in the expansion, and thus lead to spurious (non) detection of frequency
derivatives. This may hamper the estimation of the parameters of the system, in
particular their masses and distances. However, we find that overall detection
is still secured as signals still can be described by general templates. We
conclude that a better modelling of the effects of short term variations is
needed to prepare the community for astrophysical evaluations of real
gravitational wave data of AM CVn systems.
