Anomalous 14N/15N ratio in comets 122P/1995 S1 (de Vico) and 153P/2002 C1 (Ikeya-Zhang)

High-resolution (R 60,000) spectra of the Halley-type comet 122P/1995 S1 de Vico (Period 74 years) and the ``intermediate-period" comet 153P/2002 C1 Ikeya-Zhang (P 370 yr) were obtained with the 2DCoude spectrograph at the 2.7-m Harlan J. Smith telescope of the McDonald Observatory. These comets were within 1 AU from the Sun (0.66 and 0.92 AU, respectively). The spectra display CN (0,0) bands indicating an anomalous [SUP]14[/SUP]N/[SUP]15[/SUP]N isotope ratio, in the sense that it is close to half the value in Earth's atmosphere (the so-called ``Solar system value") and about half that obtained in comet Hale-Bopp from sub-millimeter measurements on HCN, generally believed to be the main parent of CN. Similar conclusions had been reached for C/1995 O1 (Hale-Bopp), an ``old long-period", and for C/2000 WM1 (LINEAR), a ``young long-period" comet. As suggested by Arpigny et al. (2003), this apparent abundance anomaly points towards the existence of other unknown parent(s) of CN, with an even higher [SUP]15[/SUP]N excess. Organic compounds like those found in interplanetary dust particles (IDPs) are good candidates. The similarity between comets of different dynamical ages is pointed out. 1 Research Director FNRS (Belgium) 2 Research Associate FNRS (Belgium)