  We use a Mach-Zehnder quantum Hall interferometer of a novel design to
investigate the interference effects at fractional filling factors. Our device
brings together the advantages of usual Mach-Zehnder and Fabry-Perot quantum
Hall interferometers. It realizes the simplest-for-analysis Mach-Zehnder
interference scheme, free from Coulomb blockade effects. By contrast to the
standard Mach-Zehnder realization, our device does not contain an etched region
inside the interference loop. For the first time for Mach-Zehnder interference
scheme, the device demonstrates interference oscillations with
$\Phi^*=e/e^*\Phi_0=\Phi_0/\nu$ periodicity at fractional filling factor 1/3.
This result indicates that we observe clear evidence for fractionally charged
quasiparticles from simple Aharonov-Bohm interference.
