  We theoretically study the linear transmission of linearly polarizedlight
pulses in an ensemble of cold atoms submitted to a static magneticfield
parallel to the direction of propagation. The carrier frequencyof the incident
pulses coincides with a resonance frequency of theatoms. The transmitted light,
the electric field of which is transversal,is examined in the polarizations
parallel and perpendicular to thatof the incident pulses. We give explicit
analytic expressions for thetransfer functions of the system for both
polarizations and for thecorresponding group delays. We demonstrate that slow
light can beobserved in a polarization, whereas fast light is simultaneously
observedin the perpendicular polarization. Moreover, we point out that, dueto
the polarization post selection, the system is not necessarilyminimum phase
shift. Slow light can then be obtained in situationswhere an irrelevant
application of the Kramers-Kronig relations could lead oneto expect fast light.
When the incident light is step modulated, wefinally show that, in suitable
conditions, the system enables oneto separate optical precursor and main field.
