  We study the properties of walls of marginal stability for BPS decays in a
class of N=2 theories. These theories arise in N=2 string compactifications
obtained as freely acting orbifolds of N=4 theories, such theories include the
STU model and the FHSV model. The cross sections of these walls for a generic
decay in the axion-dilaton plane reduce to lines or circles. From the
continuity properties of walls of marginal stability we show that central
charges of BPS states do not vanish in the interior of the moduli space. Given
a charge vector of a BPS state corresponding to a large black hole in these
theories, we show that all walls of marginal stability intersect at the same
point in the lower half of the axion-dilaton plane. We isolate a class of
decays whose walls of marginal stability always lie in a region bounded by
walls formed by decays to small black holes. This enables us to isolate a
region in moduli space for which no decays occur within this class. We then
study entropy enigma decays for such models and show that for generic values of
the moduli, that is when moduli are of order one compared to the charges,
entropy enigma decays do not occur in these models.
