We have discovered a substantial increase in the K-band brightness of a young, deeply embedded star in the Serpens NW star-forming region between 1994 August and 1995 July. The photometric history suggests a similarity to FUor or EXor outbursts. However, the star is more deeply embedded than any previous examples of those two types of eruptive events. Even in its bright state, the object is invisible in J, only scattered radiation is seen in H, the K band is still dominated by scattered light, and only in L' and longer wavelengths do we see a pointlike source. The Serpens Deeply Embedded Outburst Star (DEOS) has been detected at 800 μm, confirming its very young age. Its spectrum between 2.0 and 2.5 μm is a very steep, pure continuum, steeper than any published spectrum of a young embedded star. Most likely, the spectrum is thermal radiation from dust enshrouding the young star that completely veils any of the spectral features characterizing less obscured FUors and EXors stars. The outburst luminosity is only ≈ 15 solar luminosity, so that the preoutburst luminosity must have been very low. The Serpens DEOS is probably a young star of very low mass.
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