  A new Monte-Carlo algorithm for calculating time-dependent radiative-transfer
under the assumption of LTE is presented. Unlike flux-limited diffusion the
method is polychromatic, includes scattering, and is able to treat the
optically thick and free-streaming regimes simultaneously. The algorithm is
tested on a variety of 1-d and 2-d problems, and good agreement with benchmark
solutions is found. The method is used to calculate the time-varying spectral
energy distribution from a circumstellar disc illuminated by a protostar whose
accretion luminosity is varying. It is shown that the time lag between the
optical variability and the infrared variability results from a combination of
the photon travel time and the thermal response in the disc, and that the lag
is an approximately linear function of wavelength.
