The active galaxies have been chosen with the following criteria: (a) Seyfert 1 or 2 from the Véron-Cetty & Véron (1993) catalogue; (b) with morphological information in the RC3 Catalogue; (c) isolated, in the sense of not having a companion within 0.4 Mpc (H₀=75km s^-1 Mpc) and cz<500 km s^-1, or companions catalogued by Nilson without known z; (d) nearby, cz<6000 km s^-1; and (e) intermediate inclination (30 to 65 $^\circ$ ). The control sample galaxies have been selected among spirals verifying the same conditions (b), (c), (d) and (e), and with types similar to those of the active spirals. Thus, all the galaxies in our sample are isolated, in the sense of avoiding possible effects of interactions with luminous nearby galaxies.

The main properties of the sample galaxies, as given in the RC3 catalogue are presented in Table 1.

**Table 1:** Data for the sample galaxies in the RC3 catalogue
Name	$\alpha$ (2000.0)	$\delta$ (2000.0)	PGC	B	D₂₅	R₂₅	PA	cz	t	Seyfert
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)

Active galaxies

UGC 1395	01 55 22.2	+06 36 41	7164	14.18	1.10	0.10	--	5184	.SAT3..	1
IC 184	01 59 51.2	-06 50 28	7554	14.66	1.02	0.31	7	5400	.SBR1*.	2
IC 1816	02 31 51.3	-36 40 17	9634	13.83	1.16	0.07	--	5086	.SBR2P?	1
UGC 3223	04 59 09.4	+04 58 31	16482	13.83	1.15	0.25	80	4723	.SB.1..	1
NGC 2639	08 43 38.0	+50 12 24	24506	12.59	1.26	0.22	140	3198	RSAR1*	1
IC 2510	09 47 43.2	-32 50 15	28147	14	1.10	0.27	148	2700	.SBT2*.	2
NGC 3281	10 31 52.2	-34 51 27	31090	12.60	1.52	0.30	140	3439	.SAS2P*	2
NGC 3660	11 23 32.3	-08 39 29	34980	15	1.43	0.09	115	3678	.SBR4..	2
NGC 4253	12 18 26.4	+29 48 48	39525	14.00	0.98	0.06	--	3819	PSBS1*.	2
NGC 4507	12 35 37.1	-39 54 31	41960	12.78	1.22	0.10	--	3499	PSXT3..	2
NGC 4785	12 53 26.9	-48 44 58	43791	13.21	1.29	0.31	81	3735	PSBR3*.	2
NGC 5347	13 53 18.6	+33 29 32	49342	13.16	1.23	0.10	130	2312	PSBT2..	2
NGC 5728	14 42 24.1	-17 15 12	52521	12.37	1.49	0.24	0	2885	.SXR1*.	1
ESO 139-12	17 37 39.5	-59 56 29	60594	13.59	1.20	0.11	35	5200	PSAT4P*	2
NGC 6814	19 42 40.7	-10 19 25	63545	11.85	1.48	0.03	--	1509	.SXT4..	1
NGC 6860	20 08 46.2	-61 05 56	64166	13.68	1.13	0.25	34	4462	PSBR3..	1
NGC 6890	20 18 17.9	-44 48 25	64446	13.05	1.19	0.10	152	2471	.SAT3..	2
NGC 6951	20 37 15.2	+66 06 22	65086	11.91	1.59	0.08	170	1331	.SXT4..	2

Non-active galaxies

NGC 151	00 34 02.8	-09 42 20	2035	12.23	1.57	0.34	75	3654	.SBR4..
IC 454	06 51 06.6	+12 55 19	19725	-	1.24	0.28	140	3945	.SB.2..
NGC 2712	08 59 31.2	+44 54 56	25248	12.38	1.46	0.26	178	1833	.SBR3*.
NGC 2811	09 16 11.3	-16 18 47	26151	12.66	1.40	0.46	20	2514	.SBT1..
NGC 3571	11 11 30.1	-18 17 19	34028	12.99	1.48	0.46	94	3571	PSXT1*.
NGC 3835	11 44 05.6	+60 07 13	36493	13.20	1.29	0.39	60	2452	.SX.2*^a/
NGC 4162	12 11 51.8	+24 07 24	38851	12.55	1.37	0.22	174	2542	RSAT4..
NGC 4290	12 20 48.4	+58 05 32	39859	12.66	1.37	0.16	90	3035	.SBT2*.
NGC 4779	12 53 50.8	+09 42 33	43837	12.91	1.33	0.07	70	2793	.SBT4..
NGC 6012	15 54 13.5	+14 36 08	56334	12.96	1.32	0.14	168	1988	RSBR2*.
NGC 6155	16 26 08.4	+48 21 58	58115	13.20	1.13	0.16	145	2429	.SXT..^b

(1) Most common galaxy name;
(2) and (3) $\alpha$ and $\delta$ coordinates (equinox 2000.0);
(4) PGC number;
(5) B magnitude;
(6) isophotal diameter (at B=25 mag arcsec^-2), D₂₅, in units of log(0.1 $\times a$ (arcmin));
(7) axis ratio, R₂₅ in units of log(a/b);
(8) major axis position angle in degrees from N to E (when available);
(9) velocity in km s^-1;
(10) galaxy morphological type as given by the RC3, except for ^a(.S..2* in the RC3) and ^b(.S?... in the RC3), for which the morphological type is from Márquez & Moles (1996);
(11) Seyfert activity class for active galaxies. The data in Cols. 5-9 were taken from the RC3.

The samples are not large enough to give statistically significant results, but the detailed study of each of the galaxies in these samples should give us at least hints on what differences between active and non-active isolated spiral galaxies, if any, are to be taken into account to identify the conditions for the onset of nuclear activity.

The active sample extends farther in redshift. In particular, there are six active galaxies with cz> 4000 km s^-1, but none in the control sample. Although this changes the physical resolution achieved for those objects, we don't expect any bias due to that difference. Absolute mean B luminosities give $M_B=-20.4 \pm 0.6$ for Seyferts and $M_B=-20.3 \pm 0.9$ for non-active galaxies. Some contribution from the active galaxies would be expected as a consequence of the presence of the active nucleus itself and of the higher levels of star formation sometimes observed in that kind of galaxy (Maiolino & Rieke 1995). If any, this effect is not systematically present in our small samples, which can therefore be considered as comparable with respect to the host galaxy luminosity. The distribution of inclinations is almost flat for the non-active galaxies, whereas there is a deficiency of active galaxies at large inclinations, as expected for an optically selected sample of active galaxies (McLeod & Rieke 1995). The morphological types of spiral active galaxies are earlier than Sbc, resembling the type distribution of active galaxies in the Véron-Cetty & Véron catalogue (Moles et al. 1995); non-active galaxies have been selected to have similar types. Looking at the presence of bars in the galaxies of both samples (as catalogued in the RC3), we notice that only one galaxy in the list of non-active objects is unbarred, and three more are of intermediate type X. For the active galaxies, half are barred and four more of type X. As already quoted, the presence of a primary bar does not seem to be the direct cause for the nuclear activity. Otherwise the lists of active and non active galaxies we have observed are rather similar in global properties. We therefore estimate that the sample of non-active galaxies is well suited to be used as a control sample.

2 The sample