  Context: The link between the duration of GRBs and the nature of their
progenitors remains disputed. Short bursts (with durations of less than ~2 s)
are less frequently observed, technically more difficult to localize, and
exhibit significantly fainter afterglows. Aims: It is of critical importance to
establish whether the burst duration can reliably distinguish the different GRB
population models of collapsars and compact stellar mergers. The Swift GRB
090426 provides an unique opportunity to address this question. Its duration
(T_90=1.28 s) places GRB 090426 firmly in the short burst population, while the
high redshift (z=2.609), host galaxy properties, and prompt emission spectral
characteristics are more similar to those of long-duration GRBs. Methods: On
the basis of data obtained with the Tautenburg 2m telescope (Germany) and the
7-channel imager GROND (La Silla, Chile), we compiled the most finely sampled
light curve available for a short burst optical/NIR afterglow. The light curve
was then analysed in a standard fashion. GROND and XRT data were used to
determine the broad-band spectral energy distribution of the afterglow across
more than three orders of magnitude. Results: Our data show that a light curve
break exists at 0.4 days, which is followed by a steep decay. This light curve
decay is achromatic in the optical/NIR bands, and interpreted as a post-jet
break phase. The X-ray data do not disagree with this interpretation.
Conclusions: The half-opening angle of the suspected jet as well as the
luminosity of the optical afterglow provide additional evidence that GRB 090426
is probably linked to the death of a massive star rather than to the merger of
two compact objects.
