  We investigate a sample of 6 Herbig Ae/Be stars belonging to the Orion OB1
association, as well as 73 low mass objects, members of the $\sigma$ Orionis
cluster, in order to explore the angular momentum evolution at early stages of
evolution, and its possible connection with main-sequence Ap/Bp magnetic stars.
Using FIES and HECTOCHELLE spectra, we obtain projected rotational velocities
through two independent methods. Individual masses, radii, and ages were
computed using evolutionary models, distance, and cluster extinction. Under the
assumption that similar physical processes operate in both, T Tauri and Herbig
Ae/Be stars, we construct snapshots of the protostar's rotation against mass
during the first 10 Myr with the aid of a rotational model that includes a
variable disc lifetime, changes in the stellar moment of inertia, a dipolar
magnetic field with variable strength, and angular momentum loss through
stellar winds powered by accretion. We use these snapshots, as well as the
rotational data, to infer a plausible scenario for the angular momentum
evolution. We find that magnetic field strengths of a few k$G$ at 3 Myr are
required to match the rotational velocities of both groups of stars. Models
with masses between 2-3 $M_{\odot}$ display larger angular momentum by a factor
of $\sim 3$, in comparison to stars of similar spectral types on the
main-sequence. Even though some quantitative estimates on this dramatic
decrease with age, for Ap/Bp magnetic main-sequence stars are presented, the
results obtained for the angular momentum evolution do not explain their low
rotation.
