We present a study of 15 new brown dwarfs belonging to the \sim 7 ~ { } \mathrm { Myr } old 25 Orionis group and Orion OB1a sub-association with spectral types between M6 and M9 and estimated masses between \sim 0.07 ~ { } \mathrm { M } _ { \odot } and \sim 0.01 ~ { } \mathrm { M } _ { \odot } .
By comparing them through a Bayesian method with low mass stars ( 0.8 \raisebox { -2.15 pt } { $ \stackrel { \textstyle < } { \sim } $ } \mathrm { M } / \mathrm { M } % _ { \odot } \raisebox { -2.15 pt } { $ \stackrel { \textstyle < } { \sim } $ } 0.1 ) from previous works in the 25 Orionis group , we found statistically significant differences in the number fraction of classical T Tauri stars , weak T Tauri stars , class II , evolved discs and purely photospheric emitters at both sides of the sub-stellar mass limit .
Particularly we found a fraction of 3.9 ^ { +2.4 } _ { -1.6 } ~ { } \mathrm { \% } low mass stars classified as CTTS and class II or evolved discs , against a fraction of 33.3 ^ { +10.8 } _ { -9.8 } ~ { } \mathrm { \% } in the sub-stellar mass domain .
Our results support the suggested scenario in which the dissipation of discs is less efficient for decreasing mass of the central object .