  We present the results of a survey of the low mass star and brown dwarf
population of the 25 Orionis group. Using optical photometry from the CIDA Deep
Survey of Orion, near IR photometry from the Visible and Infrared Survey
Telescope for Astronomy and low resolution spectroscopy obtained with Hectospec
at the MMT, we selected 1246 photometric candidates to low mass stars and brown
dwarfs with estimated masses within $0.02 \lesssim M/M_\odot \lesssim 0.8$ and
spectroscopically confirmed a sample of 77 low mass stars as new members of the
cluster with a mean age of $\sim$7 Myr. We have obtained a system initial mass
function of the group that can be well described by either a Kroupa power-law
function with indices $\alpha_3=-1.73\pm0.31$ and $\alpha_2=0.68\pm0.41$ in the
mass ranges $0.03\leq M/M_\odot\leq 0.08$ and $0.08\leq M/M_\odot\leq0.5$
respectively, or a Scalo log-normal function with coefficients
$m_c=0.21^{+0.02}_{-0.02}$ and $\sigma=0.36\pm0.03$ in the mass range $0.03\leq
M/M_\odot\leq0.8$. From the analysis of the spatial distribution of this
numerous candidate sample, we have confirmed the East-West elongation of the 25
Orionis group observed in previous works, and rule out a possible southern
extension of the group. We find that the spatial distributions of low mass
stars and brown dwarfs in 25 Orionis are statistically indistinguishable.
Finally, we found that the fraction of brown dwarfs showing IR excesses is
higher than for low mass stars, supporting the scenario in which the evolution
of circumstellar discs around the least massive objects could be more
prolonged.
