Theory and observations of negative preflares in UV Cet stars

The preflare depression of light below the quiescent level may be a consequence of a weak impulsive heating of the stellar atmosphere. This anomalous response is due to the strong temperature dependence of the ionization of metals, the main source of free electrons. As a result even a small heating leads to noticeable increase of the H- opacity, that in turn leads to the temporary decrease of the flux. New photoelectric observations confirm the proposed model. (Auth.)