Whether a dwarf spheroidal galaxy is in equilibrium or being tidally disrupted by the Milky Way is an important question for the study of its dark matter content and distribution. This question is investigated using 328 recent observations on velocity and position of stars from the dwarf spheroidal Leo I. For Leo I, tidal disruption is detected, at least for stars sufficiently far from the center, but the effect appears to be quite modest.

A complicating feature of the data is that not all stars in the sample are really members of the galaxy. Some are foreground stars, located along our line of sight toward Leo I. We develop an algorithm for estimating parameters of a distribution sampled with contamination. We employ a statistical technique known as "expectation maximization" (EM).