  Context. The Vela Molecular Ridge hosts a number of young embedded star
clusters in the same evolutionary stage. Aims. The main aim of the present work
is testing whether the fraction of members with a circumstellar disk in a
sample of clusters in the cloud D of the Vela Molecular Ridge, is consistent
with relations derived for larger samples of star clusters with an age spread.
Besides, we want to constrain the age of the young embedded star clusters
associated with cloud D. Methods. We carried out L (3.78 microns) photometry on
images of six young embedded star clusters associated with cloud D of the Vela
Molecular Ridge, taken with ISAAC at the VLT. These data are complemented with
the available HKs photometry. The 6 clusters are roughly of the same size and
appear to be in the same evolutionary stage. The fraction of stars with a
circumstellar disk was measured in each cluster by counting the fraction of
sources displaying a NIR excess in colour-colour (HKsL) diagrams. Results. The
L photometry allowed us to identify the NIR counterparts of the IRAS sources
associated with the clusters. The fraction of stars with a circumstellar disk
appears to be constant within errors for the 6 clusters. There is a hint that
this is lower for the most massive stars. The age of the clusters is
constrained to ~1-2 Myr. Conclusions. The fraction of stars with a
circumstellar disk in the observed sample is consistent with the relations
derived from larger samples of star clusters and with other age estimates for
cloud D. The fraction may be lower for the most massive stars. Our results
agree with a scenario where all intermediate and low-mass stars form with a
disk, whose lifetime is shorter for higher mass stars.
