  We present a new, simple method to predict activity-induced radial velocity
variations using high-precision time-series photometry. It is based on insights
from a simple spot model, has only two free parameters (one of which can be
estimated from the light curve) and does not require knowledge of the stellar
rotation period. We test the method on simulated data and illustrate its
performance by applying it to MOST/SOPHIE observations of the planet host-star
HD189733, where it gives almost identical results to much more sophisticated,
but highly degenerate models, and synthetic data for the Sun, where we
demonstrate that it can reproduce variations well below the m/s level. We also
apply it to Quarter 1 data for Kepler transit candidate host stars, where it
can be used to estimate RV variations down to the 2-3m/s level, and show that
RV amplitudes above that level may be expected for approximately two thirds of
the candidates we examined.
