  We study the origin and properties of the population of unbound stars in the
kinematic samples of dwarf spheroidal galaxies. For this purpose we have run a
high resolution N-body simulation of a two-component dwarf galaxy orbiting in a
Milky Way potential. The dwarf is placed on a highly eccentric orbit, its
initial stellar component is in the form of an exponential disk and it has a
NFW-like dark matter halo. After 10 Gyrs of evolution the dwarf produces a
spheroidal stellar component and is strongly tidally stripped so that mass
follows light and the stars are on almost isotropic orbits. From this final
state, we create mock kinematic data sets for 200 stars by observing the dwarf
in different directions. We find that when the dwarf is observed along the
tidal tails the kinematic samples are strongly contaminated by unbound stars
from the tails. We also study another source of possible contamination by
adding stars from the Milky Way. We demonstrate that most of the unbound stars
can be removed by the method of interloper rejection proposed by den Hartog &
Katgert. We model the cleaned-up kinematic samples using solutions of the Jeans
equation with constant mass-to-light ratio and velocity anisotropy parameter.
We show that the Jeans analysis, when applied to cleaned samples, allows us to
reproduce the mass and mass-to-light ratio of the dwarf with accuracy typically
better than 25 percent. The analysis was applied to the new data for the Fornax
dSph galaxy. We show that after careful removal of interlopers the velocity
dispersion profile of Fornax can be reproduced by a model in which mass traces
light with a mass-to-light ratio of 11 solar units and isotropic orbits. We
demonstrate that most of the contamination in the kinematic sample of Fornax
probably originates from the Milky Way. (Abridged)
