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The Orion OB1a sub-association is a rich low mass star (LMS) region. Previousspectroscopic studies have confirmed 160 LMSs in the 25 Orionis stellar group(25 Ori), which is the most prominent overdensity of Orion OB1a. Nonetheless,the current census of the 25 Ori members is estimated to be less than 50%complete, leaving a large number of members to be still confirmed. We retrieved172 low-resolution stellar spectra in Orion OB1a observed as ancillary sciencein the SDSS-III/BOSS survey, for which we classified their spectral types anddetermined physical parameters. To determine memberships, we analyzed theH$_\alpha$ emission, LiI$\lambda$6708 absorption, and NaI$\lambda\lambda$8183,8195 absorption as youth indicators in stars classified as M-type. We report 50new LMSs spread across the 25 Orionis, ASCC 18, and ASCC 20 stellar groups withspectral types from M0 to M6, corresponding to a mass range of 0.10$\lem/\textrm{M}_\odot \le$0.58. This represents an increase of 50% in the numberof known LMSs in the area and a net increase of 20% in the number of 25 Orimembers in this mass range. Using parallax values from the Gaia DR1 catalog, weestimated the distances to these three stellar groups and found that they areall co-distant, at 338$\pm$66 pc. We analyzed the spectral energy distributionsof these LMSs and classified their disks by evolutionary classes. Using H-Rdiagrams, we found a suggestion that 25 Ori could be slightly older that theother two observed groups in Orion OB1a.

New Low-Mass Stars in the 25 Orionis Stellar Group and Orion OB1a Sub-association from SDSS-III/BOSS Spectroscopy