  A study of secondary instabilities in ac-driven electroconvection of a
planarly aligned nematic liquid crystal is presented. At low frequencies one
has a transition from normal rolls to a zig-zag pattern and at high frequencies
a direct transition from normal to abnormal rolls. The crossover defines a
codimension-2 point. This point is also the origin of a line of homoclinic
bifurcations, which enable the transition from zig-zags to abnormal rolls at
lower frequencies.
