We experimentally observed patterns consisting of domains of upflow hexagons coexisting with domains of downflow hexagons in Boussinesq Rayleigh-B\'enard convection. The sequence of patterns, as the control parameter ε0ex0ex=0ex0exΔT/ΔTc-1 is increased, is from ordered or disordered rolls at onset to extended targets and spirals at ε~1, to coexisting hexagons at ε~3. Hexagons occur for all values of the Prandtl number 2.8≤P≤28 investigated. Surprisingly, they appear in a range where only rolls were known to be stable, and their wave number differs substantially from the roll wave number they coexist with.
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Assenheimer et al. (1996) studied this question.