– 15 – 4.1.2. Pre-main-sequence stars identification for K and earlier type stars. Background objects are known to contaminate samples of Spitzer -selected YSO candidates (Harvey et al. 2007, Oliveira et al. 2009). At low flux levels, background galaxies are the main source of contamination. However, the optical and near-IR flux cuts we have implemented as part of our sample selection criteria seem to have very efficiently removed any extragalactic sources that could remain in the Cores to Disks catalog of Ophiuchus YSO candidates. At the bright end of the flux distribution, Asymptotic Giant Branch (AGB) stars and classical Be stars are the main source of contamination. AGB stars are surrounded by shells of dust and thus have small, but detectable, IR excesses. The extreme luminosites ( ∼ 10 4 L$_{⊙}$) of AGB stars imply that they can pass our optical and near-IR flux cuts even if they are located several kpc away. Classical Be stars are surrounded by a gaseous circumstellar disk that is not related to the primordial accretion stage but to the rapid rotation of the object (Porter & Rivinius, 2003). Classical Be stars exhibit both IR excess (from free-free emission) and H α emission (from the recombination of the ionized hydrogen in the disk) and thus can easily be confused with Herbig Ae/Be stars (early-type PMS stars). There is only one B-type star in our target list, object # 2. This object is located close to the eastern edge of the Cores to Disks maps of Ophiuchus, shows very little 24 µ m excess, and is not detected at 70 µ m. Its weak IR excess is more consistent with the free-free emission of a classical Be star than with the thermal IR excess produced by circumstellar dust around a Herbig Ae/Be star. Since there is no clear evidence that object # 2 is in fact a pre-main-sequence star, we do not include it in our sample of transition disks.