  The detailed spiral structure in the outer Galactic disk is still poorly
known, and for several Galactic directions we rely on model extrapolations. One
of these regions is the fourth Galactic quadrant, in the sector comprised
between Vela and Carina (270 <l< 300) where no spiral arms have been detected
so far in the optical beyond 270. By means of deep UBVI photometry, we search
for spiral features in known low absorption windows.U photometry, although
demanding, constitutes a powerful tool to detect and characterize distant
aggregates, and allows to derive firmer distance estimates. We studied a
direction close to the tangent (l=290) to the Carina arm, in an attempt to
detect optical spiral tracers beyond the Carina branch, where radio
observations and models predictions indicate the presence of the extension of
the Perseus and Norma-Cygnus spiral arms in the fourth quadrant.Along this line
of sight, we detect three distinct groups of young stars. Two of them, at 2.5
and 6.0 kpc, belong to the Carina spiral arm (which is crossed twice in this
particular direction).The latter is here detected for the first time. The third
group, at a distance of 12.7 kpc, is part of the Perseus arm which lies beyond
the Carina arm, and constitutes the first optical detection of this arm in the
fourth Galactic quadrant. The position of this feature is compatible with HI
observations and model predictions. We furthermore present evidence that this
extremely distant group, formerly thought to be a star cluster (Shorlin 1), is
in fact a diffuse young population. In addition, our data-set does not support
the possible presence of the Monoceros Ring toward this direction. This study
highlights how multicolor optical studies can be effective to probe the spiral
structure in the outer Galactic disk.
