Atomic dipole transitions involving high-nl Rydberg states are frequently encountered in astrophysics and in laser physics. In astrophysics, the interpretation of recombination spectra emitted by extraterrestrial sources requires the knowledge of a large number of transition energies and rates as emphasized for example by Hoang-Binh et al. (1987) and Hoang-Binh (1990, 1993). In laser physics, oscillator strengths for transitions with large values of the principal quantum number n can now be measured in laboratory by accurate techniques (see e.g. Connerade et al. 1992; Mende & Kock 1996).
Recently, several dozens of lines involving intermediate and relatively large (n, l) levels in the low ionization stages of very common elements such as carbon, nitrogen and oxygen were observed in the spectrum of the planetary nebula NGC 7027 between 399 and 1050 nm (Péquignot & Baluteau 1994; Baluteau et al. 1995; Péquignot 1996, 1997). More particularly, owing to the large abundance of carbon in NGC 7027, several Rydberg series of C II, C III and C IV were probably detected up to principal quantum number n = 30 by Baluteau et al. (1995) and Péquignot (1996, 1997). In these papers, it is stated that available compilations of high-nl atomic energy levels may not be sufficiently comprehensive to fully exploit the nebular spectroscopic data now accessible to common astrophysical devices and this lack of basic atomic data is pointed out as a major limitation to future progress in the field of nebulae.
Up to now, the highest (l 
3) Rydberg terms experimentally determined were
1s22s27f 2F
, 8g 2G, 7h 2H in C II, 1s22s7f 3F,
7g 3G, 6h 3H in C III (Bashkin & Stoner 1975) and 1s28f 2F,
8g 2G, 9h 2H, 9i 2I in C IV (Bashkin & Stoner 1975;
Tunklev et al. 1997). The aim of the present work is to provide predicted
term energies for values of the principal quantum number up to 30 and
orbital quantum numbers l 3 in these three ions using the Ritz
and the polarization formulae.
At the same time, wavelength predictions to an accuracy of at least
0.01 nm for lines involving high-nl Rydberg states together with
the corresponding oscillator strengths, calculated with the help of
the relativistic Hartree-Fock (HFR) method, are reported in the region
400-1100 nm which might be useful for the interpretation of astronomical
observations.