Almost thirty years ago, Walborn (1973) and Conti & Leep (1974) called attention to a group of Of stars where the He II $\lambda$ 4686 emission line is found to display a double-peaked profile. These objects were classified as Onfp stars by Walborn (1973), whereas Conti & Leep (1974) favored an Oef naming (we shall adopt the latter terminology throughout this paper). In both papers, it was pointed out that the stars in this category are rapid rotators (as derived from their broad absorption lines) and that the occurrence of the Oef phenomenon may be related to rotation. More specifically, Conti & Leep (1974) suggested that the peculiar morphology of the He II $\lambda$ 4686 profiles results from a combination of a roughly central emission and a slightly blueshifted absorption in stars having stellar winds with appreciable rotational rates (see also Petrenz & Puls 1996).

Oef stars usually do not display emission in the Balmer lines, but they have broad absorption lines indicative of large projected rotational velocities. All known Oef stars are of early spectral type. Conti & Leep (1974) listed a total of six stars belonging to this category; all of them, except $\zeta$ Pup, being located in the northern hemisphere. The brightest and hence best studied representatives of this group are $\zeta$ Pup (O4 ef) and $\lambda$ Cep (O6 ef). The remaining four objects (HD 14434, HD 14442, HD 192281 and BD $+60^{\circ }$ 2522) have received little attention over the last three decades.

Considerable progress in our understanding of the stellar winds of early-type stars has been achieved through extensive monitoring of their spectroscopic variability and the discovery that some of the cyclical variations could be related to a rotational modulation of the stellar wind (for a review, see e.g. Fullerton 1999). Since rotation is believed to shape the winds of Oef stars, these objects appear a priori as good candidates to search for a rotational wind modulation. In fact, part of the line profile variability observed in the spectra of $\zeta$ Pup and $\lambda$ Cep has been attributed to the effect of rotation (Moffat & Michaud 1981; Howarth et al. 1995; Kaper et al. 1999). In order to investigate the spectroscopic variability of the other, poorly known, Oef stars, we have started an extensive observing campaign to collect spectra of these objects. In the present paper, we focus on the results of our seven year observing campaign of BD $+60^{\circ }$ 2522. The spectroscopic variability of HD 14434, HD 14442 and HD 192281 will be discussed in a forthcoming paper (De Becker et al. 2003, in preparation).

BD $+60^{\circ }$ 2522 (V = 8.7) is the ionizing star of NGC 7635, the so-called "Bubble Nebula'' (see e.g. Christopoulou et al. 1995). NGC 7635 lies at the edge of a low-density clumpy molecular cloud and the nebula can be interpreted as a wind-blown bubble created by the interaction of the stellar wind of BD $+60^{\circ }$ 2522 with the ambient interstellar medium (Christopoulou et al. 1995; Chavarría-K. et al. 1987). While many investigations have focused on the nebula, little attention has been paid to the star itself.

The paper is organized as follows. Section 2 briefly summarizes our observing campaign and the data reduction. In Sect. 3 we present the results of our radial velocity and line profile variability analyses. The origin of the line profile variability is discussed in Sect. 4 and our conclusions are highlighted in Sect. 5.

Table 1: Overview of our observing campaigns. The third column indicates the detector used with the Aurélie spectrograph (see text). N yields the number of spectra. $\Delta T$ , $\overline {\Delta t}$ , $\Delta~\nu_{\rm nat}$ and $\nu _{\rm max}$ provide for each campaign respectively the total time elapsed between the first and the last observation, the average time interval between consecutive exposures during the same night, the natural width of a peak in the power spectrum and the highest frequency that can be sampled with our data set (see text). (S/N)₀ stands for the average signal to noise ratio evaluated over a line-free region of the spectrum.
Run Epoch Detector Spectral range Recipr. disp. N $\Delta T$ $\overline {\Delta t}$ $\Delta~\nu_{\rm nat}$ $\nu _{\rm max}$ (S/N)₀

(Å) (Å mm^-1) days days (d^-1) (d^-1)

[1] Aug. 1996 TH 7832 4100-4950 33 13 3.124 0.051 0.320 9.8 320

[2] Jul. 1997 TH 7832 4100-4950 33 18 6.109 0.044 0.164 11.4 430

[3] Sep. 1998 TH 7832 4455-4905 16 6 9.008 0.058 0.111 8.6 230

[4] Jul. 1999 TH 7832 4100-4950 33 14 6.029 0.040 0.166 12.5 215

[5] Aug. 1999 TH 7832 4100-4950 33 12 13.008 - 0.077 1.0 210

[6] Sep. 2000 CCD 4455-4905 16 18 10.998 0.024 0.091 20.8 470

[7] Sep. 2001 CCD 4455-4905 16 14 7.053 0.026 0.142 19.2 320

[8] Sep. 2002 CCD 4455-4905 16 9 5.006 0.028 0.200 17.6 380

Run	Epoch	Detector	Spectral range	Recipr. disp.	N	$\Delta T$	$\overline {\Delta t}$	$\Delta~\nu_{\rm nat}$	$\nu _{\rm max}$	(S/N)₀
			(Å)	(Å mm^-1)		days	days	(d^-1)	(d^-1)
[1]	Aug. 1996	TH 7832	4100-4950	33	13	3.124	0.051	0.320	9.8	320
[2]	Jul. 1997	TH 7832	4100-4950	33	18	6.109	0.044	0.164	11.4	430
[3]	Sep. 1998	TH 7832	4455-4905	16	6	9.008	0.058	0.111	8.6	230
[4]	Jul. 1999	TH 7832	4100-4950	33	14	6.029	0.040	0.166	12.5	215
[5]	Aug. 1999	TH 7832	4100-4950	33	12	13.008	-	0.077	1.0	210
[6]	Sep. 2000	CCD	4455-4905	16	18	10.998	0.024	0.091	20.8	470
[7]	Sep. 2001	CCD	4455-4905	16	14	7.053	0.026	0.142	19.2	320
[8]	Sep. 2002	CCD	4455-4905	16	9	5.006	0.028	0.200	17.6	380

1 Introduction