A search for highly reddened early-type stars: Optical photometry and spectroscopy of stars in the Stephenson objective prism survey

The Stephenson objective prism survey contains many stars of apparently early spectral type whose colors imply interstellar extinction values that lie in the key range (8 < A(V) < 30) between those of nearby reddened stars and the Galactic center infrared sources. As part of a wider program to determine the properties of objects in this survey, we present new optical spectroscopy for 45% of the catalog of similar to 440 stars, placing constraints on their spectral types and luminosity classes. Contamination by intrinsically red stars, suggested by previous infrared observations of much smaller subsamples, is shown to be heavy: around 90% of the survey appears to consist of late-type, mostly K and M, stars of luminosity class I, II, or III. We also identify six S stars, one of which has high photospheric lithium abundance. Fifteen highly reddened early-type stars are identified: these have visual extinctions in the range 6 < A(V) < 16 mag, confirming their potential for studies of the interstellar medium.