  Benard-Marangoni convection in a system of two superimposed liquids is
investigated theoretically. Extending previous studies the complete
hydrodynamics of both layers is treated and buoyancy is consistently taken into
account. The planform selection problem between rolls, squares and hexagons is
investigated by explicitly calculating the coefficients of an appropriate
amplitude equation from the parameters of the fluids. The results are compared
with recent experiments on two-layer systems in which squares at onset have
been reported.
