  Premixed flames propagating in horizontal tubes are observed to take on shape
convex towards the fresh mixture, which is commonly explained as a buoyancy
effect. A recent rigorous analysis has shown, on the contrary, that this
process is driven by the balance of vorticity generated by a curved flame front
with the baroclinic vorticity, and predicted existence of a regime in which the
leading edge of the flame front is concave. We report first experimental
realization of this regime. Our experiments on ethane and n-butane mixtures
with air show that flames with an inflection point on the front are regularly
produced in lean mixtures, provided that a sufficiently weak ignition is used.
The observed flame shape perfectly agrees with the theoretically predicted.
