The new carbon symbiotic star IPHAS J205836.43+503307.2

Aims. We are performing a search for symbiotic stars using IPHAS, the INT H alpha survey of the northern Galactic plane, and follow-up observations. Methods. Candidate symbiotic stars are selected on the basis of their IPHAS and near-IR colours, and spectroscopy and photometry are obtained to determine their nature. We present here observations of the symbiotic star candidate IPHAS J205836.43+503307.2. Results. The optical spectrum shows the combination of a number of emission lines, among which are the high-excitation species of [O III], He II, [Ca v], and [Fe VII], and a red continuum with the features of a star at the cool end of the carbon star sequence. The nebular component is spatially resolved: the analysis of the spatial profile of the [N II]6583 line in the spectrum indicates a linear size of similar to 2 ''.5 along the east-west direction. Its velocity structure suggests an aspherical morphology. The near-infrared excess of the source, which was especially strong in 1999, indicated that a thick circumstellar dust shell was also present in the system. The carbon star has brightened in the last decade by two to four magnitudes at red and near-infrared wavelengths. Photometric monitoring during a period of 60 days from November 2010 to January 2011 reveals a slow luminosity decrease of 0.2 mag. Conclusions. From the observed spectrophotometric properties and variability, we conclude that the source is a new Galactic symbiotic star of the D-type, of the rare kind that contains a carbon star, likely a carbon Mira. Only two other systems of this type are known in the Galaxy.