Oh, wait, O8 de Sitter may be unstable!

Abstract: We analyze the stability of four-dimensional de Sitter vacua constructed by compactifying massive Type IIA supergravity in the presence of two O8 sources [1]. When embedded in String Theory the first source has a clear interpretation as an O8$-$ plane, but the second one could correspond to either an O8$+$ plane or to an O8$-$ plane with 16 D8-branes on top. We find that this latter solution has a tachyonic instability, corresponding to the D8-branes moving away from the O8$-$ plane. We comment on the possible ways of distinguishing between these sources.