The effects of opacity in the transition region of YZ CMi

It has generally been assumed that the emission from the upper atmosphere o f late-type stars is optically thin. In the present paper we use the Si IV and C IV resonance Lines to investigate this assumption for the active dMe star YZ CMI. The significant deviations of the line ratios from their optic ally thin values, demonstrate that opacity can be quite important particula rly in the case of stellar flares. These deviations are combined with a met hod of escape probabilities to derive optical depths of approximately unity for the lines under consideration. We demonstrate that, if the electron de nsity in the atmosphere is known, opacity can provide important information on the linear dimensions of the scattering layer. Using this technique, we have estimated path lengths of a few kilometers for one of the flares unde r consideration.