3 Observations and data reduction

A journal of the first set of the photometric observations is given in Table 1. Column (1) gives the original catalogued number, Cols. (2)-(9) give the number of frames per filter, the integration time (in seconds) and seeing conditions (in arcsec).

 

 

Table 1: Journal of observations. The number of frames per filter, the integration time (in seconds), and the mean FWHM for each observation (in arcsec) are given.

Galaxy pair	B	<B>FWHM	V	<V>FWHM	R	<R>FWHM	I	<I>FWHM
KPG64	$2\times 1200$	2.4	$3\times 180$	2.3	$5\times 90$	2.6	$5\times 90$	3.1
KPG68	$1\times 1200$	2.3	$5\times 180$	2.5	$10\times 60$	2.3	$5\times 120$	2.8
KPG75	$1\times 1800$	2.1	$2\times 600$	2.2	$3\times 240$	2.1	$3\times 180$	2.7
KPG88	$1\times 1800$	2.4	$1\times 1800$	2.5	$1\times 1200$	2.6	$1\times 900$	3.3
KPG98	$1\times 1800$	2.6	$1\times 900$	2.5	$2\times 300$	3.1	$2\times 300$	3.1
KPG102	$1\times 1800$	2.1	$1\times 1200$	2.4	$1\times 600$	2.4	$1\times 600$	2.6
KPG103	$1\times 1800$	2.7	$1\times 600$	2.5	$1\times 300$	2.5	$1\times 300$	2.8
KPG108	$1\times 1800$	2.5	$1\times 1200$	2.5	$1\times 600$	2.2	$1\times 300$	2.4
KPG112	$1\times 1800$	2.5	$3\times 180$	2.3	$5\times 60$	2.2	$5\times 60$	2.8
KPG125	$1\times 1800$	2.6	$1\times 1200$	2.9	$2\times 300$	3.2	$2\times 300$	3.6
KPG136	$1\times 1800$	2.7	$2\times 900$	2.5	$1\times 600$	2.3	$2\times 300$	2.9
KPG141	$1\times 1800$	2.4	$1\times 1200$	2.3	$1\times 600$	2.2	$1\times 600$	2.5
KPG150	$1\times 1800$	2.7	$2\times 600$	2.4	$2\times 300$	2.4	$3\times 180$	2.7
KPG151	$1\times 1800$	2.4	$1\times 1200$	2.6	$1\times 900$	2.2	$1\times 600$	3.3
KPG156	$1\times 1800$	2.6	$1\times 1200$	2.6	$1\times 600$	2.9	$1\times 600$	3.6
KPG159	$1\times 1800$	3.6	$1\times 600$	3.4	$1\times 600$	3.2	$1\times 300$	3.5
KPG160	$1\times 1800$	2.5	$2\times 600$	2.1	$3\times 240$	2.3	$3\times 240$	2.8
KPG168	$1\times 1200$	3.1	$2\times 300$	2.9	$3\times 120$	2.9	$3\times 120$	3.5
KPG195	$1\times 1200$	3.0	$2\times 300$	3.2	$5\times 90$	2.6	$5\times 90$	2.5
KPG211	$1\times 1800$	2.7	$1\times 600$	2.5	$2\times 240$	3.0	$3\times 180$	3.7
KPG216	$1\times 1800$	2.9	$1\times 1200$	2.7	$1\times 600$	2.3	$1\times 600$	2.4
KPG249	$1\times 1800$	3.0	$1\times 600$	3.3	$1\times 300$	3.5	$1\times 300$	3.4
KPG295	$1\times 1800$	3.0	$2\times 600$	3.2	$5\times 150$	3.1	$5\times 120$	2.9
KPG302	$1\times 1200$	2.3	$3\times 300$	2.3	$5\times 180$	2.2	$6\times 120$	2.7
KPG313	$1\times 1800$	3.3	$1\times 600$	3.2	$3\times 180$	3.1	$3\times 120$	2.6
KPG332	$1\times 1800$	2.9	$1\times 1200$	2.9	$2\times 300$	2.5	$2\times 240$	2.6
KPG347	$1\times 900$	2.8	$1\times 600$	3.0	$3\times 180$	2.5	$3\times 120$	2.6
KPG389	$1\times 1800$	3.5	$1\times 1200$	3.6	$2\times 600$	3.2	$2\times 300$	2.6
KPG396	$1\times 1800$	2.4	$1\times 1200$	2.7	$1\times 900$	2.5	$1\times 900$	2.7
KPG404	$1\times 900$	2.7	$2\times 300$	2.9	$5\times 150$	3.1	$5\times 120$	2.4
KPG426	$1\times 1800$	2.9	$1\times 600$	3.1	$2\times 300$	3.2	$3\times 180$	3.0
KPG440	$1\times 1200$	2.4	$1\times 900$	3.0	$2\times 300$	2.8	$2\times 240$	2.6
KPG455	$1\times 1800$	2.3	$1\times 600$	2.5	$3\times 180$	2.6	$3\times 180$	3.1

Table 2 reports some relevant information for the observed pairs coming from the literature. Column (1) is the KPG catalogued number, Col. (2) reports other identifications, Col. (3) the apparent B magnitude from the Nasa Extragalactic Database (NED), Col. (4) the linear separation (in kpc), Col. (5) the radial velocity in km s^-1 from NED, and finally, Col. (6) gives the major axis diameter (at $\mu_{B} = 25$) for each component galaxy (in kpc).

 

 

Table 2: General data for the observed galaxies.

KPG Number	Identif.	B mag	x12 (kpc)	$V_{\rm Rad}$ (km s-1)	A25 (kpc)
KPG64A	UGC 01810	13.42(p)	39.5	7563	55.8
KPG64B	UGC 01813	15.08(p)		7335	27.4
KPG68A	NGC 0935	13.63(p)	17.4	4142	29.3
KPG68B	IC 1801	14.56(p)		4023	19.3
KPG75A	UGC 02222	14.56(p)	21.5	4913	32.0
KPG75B	UGC 02225	15.21(p)		4965	18.6
KPG88A	UGC 02627	14.89(a)	32.7	4224	30.8
KPG88B	UGC 02629	15.28(p)		4128	15.1
KPG98A	UGC 02954	15.23(p)	41.2	5306	17.4
KPG98B	MRK 1081	15.15(p)		5345	16.0
KPG102A	CGCG 393-070	15.50(p)	34.4	10778	21.0
KPG102B	UGC 03136	15.00(p)		10674	34.8
KPG103A	CGCG 420-003	15.70(p)	55.2	8313	20.3
KPG103B	UGC 03179	14.46(p)		8337	28.4
KPG108A	UGC 03405	15.32(p)	33.9	3738	20.1
KPG108B	UGC 03410	14.99(p)		3921	29.8
KPG112A	UGC 03445	14.25(p)	9.7	3119	21.3
KPG112B	UGC 03446	13.86(p)		3116	21.2
KPG125A	NGC 2341	13.84(a)	50.9	5227	24.9
KPG125B	NGC 2342	13.10(a)		5276	29.1
KPG136A	CGCG 086-028	14.80(p)	37.8	9907	31.1
KPG136B	CGCG 086-029	15.00(p)		9813	32.1
KPG141A	UGC 04005	14.60(p)	89.8	5044	31.8
KPG141B	CGCG 030-014	14.80(p)		4896	14.4
KPG150A	NGC 2486	14.16(a)	99.3	4649	29.5
KPG150B	NGC 2487	13.23(a)		4841	45.6
KPG151A	UGC 04133	16.00(p)	32.5	9130	50.5
KPG151B	UGC 04134	15.37(p)		8968	32.6
KPG156A	NGC 2535	13.31(a)	27.7	4097	30.9
KPG156B	NGC 2536	14.70(a)		4142	17.0
KPG159A	CGCG 088-052	15.60(p)	26.0	5232	9.9
KPG159B	UGC 04286	14.32(p)		5143	19.9
KPG160A	NGC 2544	13.80(a)	17.4	2828	16.0
KPG160B	CGCG 331-037	15.50(p)		3589	11.0
KPG168A	NGC 2648	12.74(p)	17.6	2060	23.1
KPG168B	CGCG 060-036	15.40(p)		2115	9.2
KPG195A	NGC 2798	13.04(a)	11.5	1726	16.3
KPG195B	NGC 2799	14.32(p)		1865	11.3
KPG211A	NGC 2959	13.65(p)	26.8	4429	29.3
KPG211B	NGC 2961	15.52(p)		4501	12.2
KPG216A	NGC 3018	14.13(p)	17.8	1863	6.45
KPG216B	NGC 3023	13.50(p)		1879	14.7
KPG249A	NGC 3395	12.40(a)	8.8	1625	10.5
KPG249B	NGC 3396	12.63(p)		1625	12.3
KPG295A	NGC 3786	*13.24(p)	14.9	2678	19.2
KPG295B	NGC 3788	13.46(p)		2699	13.9
KPG302A	NGC 3893	11.16(s)	13.9	977	17.1
KPG302B	NGC 3896	13.89(p)		980	5.9
KPG313A	IC 0749	12.92(s)	10.9	784	7.5
KPG313B	IC 0750	12.94(s)		701	9.1
KPG332A	NGC 4298	12.04(s)	9.5	1135	10.3
KPG332B	NGC 4302	12.50(s)		1149	13.0
KPG347A	NGC 4567	12.06(s)	10.8	2274	20.2
KPG347B	NGC 4568	11.68(s)		2255	29.3
KPG389A	NGC 5257	*13.50(p)	36.6	6798	39.0
KPG389B	NGC 5258	*13.49(p)		6757	37.8
KPG396A	UGC 08713	15.25(p)	28.6	4956	24.3
KPG396B	UGC 08715	14.50(p)		4517	22.7

 

Table 2: continued.

KPG Number	Identif.	B mag	x12 (kpc)	$V_{\rm Rad}$ (km s-1)	A25 (kpc)
KPG404A	NGC 5394	13.70(a)	26.2	3472	22.8
KPG404B	NGC 5395	12.10(a)		3491	34.1
KPG426A	UGC 09376	14.70(p)	26.8	7676	40.6
KPG426B	CGCG 220-030	14.89(p)		7764	46.8
KPG440A	NGC 5774	12.74(s)	27.2	1567	20.9
KPG440B	NGC 5775	12.24(s)		1681	21.2
KPG455A	NGC 5857	13.86(a)	38.4	4682	21.6
KPG455B	NGC 5859	13.27(a)		4764	41.7

(a) Total (asymptotic) magnitude in the B system, derived by extrapolation from photoelectric aperture-magnitude data.
(s) Total asymptotic magnitude in the B system, derived by extrapolation from (surface) photometry with photoelectric zero point.
(p) Photographic magnitude reduced to the $B_{\rm T}$ system.

Images were debiased, trimmed, and flat-fielded using standard IRAF procedures. First, the bias level of the CCD was subtracted from all exposures. A run of 5-10 bias images was obtained per night, and these were combined into a single bias frame which was then applied to the object frames. The images were flat-fielded using sky flats taken in each filter at the beginning and/or at the end of each night.

Photometric calibration was achieved by nightly observations of standard stars of known magnitudes from the "Dipper Asterism'' M 67 star cluster (Chevalier & Ilovaisky 1991). A total of 29 standard stars with a colour range $-0.1 \leq (B-V) \leq 1.4$ and a similar range in (V-I) were observed. The principal extinction coefficients in B, V, R and I as well as the colour terms were calculated according to the following equations:

$$\begin{eqnarray*}&&\hspace*{-3.5mm}B-b = \alpha_{B} + \beta_{B}(b-v)_{0}\\ &&\h... ...)_{0}\\ &&\hspace*{-3.5mm}I-i = \alpha_{I} + \beta_{I}(v-r)_{0} \end{eqnarray*}$$

where B, V, R and I are the standard magnitudes, b, v, r and i are the instrumental (and airmass-corrected) magnitudes. $\alpha$ and $\beta$ are the transformation coefficients for each filter.

In a first iteration, a constant value associated with the sky background was subtracted using an interactive procedure that allows the user to select regions on the frame free of galaxies and bright stars. However, occasionally, at the end of the reduction procedure, we still had images with a noticeable gradient in the sky background. For these images, a fifth-order polynomial was fitted and subtracted from the entire frame. After this processing, the sky background is usually flat to a level ${\sim} 1{-}2$%. Errors in determining the sky background, are, in fact, probably the dominant source of error in the estimation of the colour and surface brightness profiles. For this reason, we decided to apply this polynomial correction to all the images in this work.

The most energetic cosmic-ray events were automatically masked using the COSMICRAYS task and field stars were removed using the IMEDIT task when necessary. Within the galaxy itself, care was taken to identify superimposed stars. A final step in the basic reduction involved registration of all available frames for each galaxy and in each filter to within $\pm 0.1$ pixel. This step was performed by measuring centroids for foreground stars on the images and then performing geometric transformations using GEOMAP and GEOTRAN tasks in IRAF.

Elliptical surface brightness contours were fitted using the STSDAS package ISOPHOTE. An initial starting guess for the ellipse-fitting routine was provided interactively by estimating points that represent the ends of the major and minor axis at an isophotal level of relatively high signal-to-noise ratio. Since we are interested on the mean global properties of these profiles and not in their detailed structure, we report azimuthally averaged profiles for spirals by fitting ellipses with a fixed position angle and ellipticity previously determined on the external isophotes of each galaxy. A more detailed analysis and interpretation will be presented in a forthcoming paper (Hernández-Toledo & Puerari, in preparation).

3.1 Errors

Total magnitudes can be calculated by analytically extrapolating a fitting of a disk beyond the outermost isophote to infinity. However, disk fitting is notoriously fraught with uncertainty (cf. Knapen & van der Kruit 1991). Alternatively, we estimate in this work a total magnitude computed from polygonal apertures chosen interactively to assure that they are large enough to contain the whole galaxy and still small enough to limit the errors due to the sky error and light contamination from a neighbor galaxy. This is achieved in each band by using polygonal apertures with and without the the sky background removed within POLYPHOT routines in IRAF. In an Appendix, we are also reporting total magnitudes at three different circular apertures by using the PHOT routines in IRAF. Foreground stars within the aperture were removed interactively. In some cases, the separation of the galaxies allowed us to model the light distribution in each galaxy and then to try an iterative subtraction as reported in Junqueira et al. (1998). In cases where this procedure was not possible, our estimations must be taken with care. See Table 3 and comments on individual objects.

An estimation of the errors in our photometry involves two parts: 1) the procedures to obtain instrumental magnitudes and 2) the uncertainty when such instrumental magnitudes are transformed to the standard system. For 1), notice that the magnitudes produced at the output of the IRAf routines (QPHOT, PHOT and POLYPHOT) have a small error that is internal for those procedures. Since we also have applied extinction corrections to the instrumental magnitudes in this step, our estimation of the errors are mainly concerned with these corrections and the estimation of the airmass. After a least square fitting, the associated errors to the slope for each principal extinction coefficient are; $\delta(k_{B}) \sim 0.038$, $\delta(k_{V}) \sim 0.035$, $\delta(k_{R}) \sim 0.020$ and $\delta(k_{I}) \sim 0.020$. An additional error $\delta({\rm airmass}) \sim 0.005$ from the airmass routines in IRAF was also considered.

For 2), the zero point and first order colour terms are the most important to consider. After transforming to the standard system, by adopting our best-fit coefficients, the formal errors from the assumed relations for $\alpha$ were 0.05, 0.04, 0.04 and 0.04 in B, V, R and I and 0.04, 0.03, 0.03 and 0.04 for $\beta$. To estimate the total error in each band, it is necessary to use the transformation equations and then propagate the errors. Total typical uncertainties are 0.15, 0.14, 0.15 and 0.14 in B, V, R and I bands, respectively.

The estimated total magnitudes in this work were compared against other external estimations reported in the literature. This has been done for: 1) the standard stars and 2) those paired galaxies in common with other works.

3.2 Standard stars

For the standard stars, a comparison of our CCD magnitudes against those reported in Chevalier & Ilovaisky (1991) for 29 stars in common, are shown in Fig. 1.

Figure 1: Comparison between our estimated magnitudes and those from Chevalier & Ilovaisky (1991) for 29 standard stars in common.

Figure 1 shows no significant deviations between our CCD magnitudes and the standard star magnitudes. According to these results, a $\sigma \sim 0.13$, or a similar value, could be expected as the typical error for our magnitude estimations in paired galaxies. This is in agreement with our error estimations.

3.3 Paired galaxies

We begin with a comparison in Fig. 2 of our total magnitudes in B and V bands and those reported in the RC3 Catalogue (de Vaucouleurs et al. 1991).

Figure 2: Comparison between our total B and V magnitudes and total magnitudes from RC3 Catalogue.

We find that, except for three galaxies (KPG347B, KPG404B and KPG440A) the agreement with our measures is reasonably good. Rms values from our comparisons are 0.17 and 0.13 mag in B and V bands respectively. However, as noted in Table 3 KPG347 and KPG404 involve two overlapping pairs (CP) where our iterative magnitude estimation procedure could produce some error. In addition, the associated errors in B and V magnitudes reported in RC3 are 0.1 mag for KPG347B and KPG440A and 0.2 mag for KPG404B.

 

 

Table 3: Magnitudes and colour indices.

KPG	B	V	R	I	B-V	B-I	$B_{\rm T}^{0}$	$(B-V)_{\rm T}^{0}$	Notes
KPG64A	13.70	12.92	12.37	11.70	0.78	2.00	13.32	0.66
KPG64B	15.17	14.21	13.66	12.98	0.96	2.18	14.44	0.75
KPG68A	13.56	12.82	12.19	11.51	0.73	2.05	13.02	0.58	CP
KPG68B	14.73	13.98	13.31	12.65	0.75	2.08	14.07	0.58	CP
KPG75A	14.62	13.68	13.19	12.18	0.95	2.44	14.07	0.80
KPG75B	15.36	14.49	13.99	12.97	0.87	2.39	14.50	0.65
KPG88A	15.06	13.69	12.90	12.17	1.37	2.89	14.13	1.15
KPG88B	16.12	14.85	14.11	13.44	1.27	2.68	15.21	1.05
KPG98A	15.62	15.34	14.08	13.28	0.28	2.34	14.76	0.10
KPG98B	15.51	14.22	13.03	12.07	1.29	3.44	14.89	1.12
KPG102A	15.68	15.18	14.84	13.87	0.51	1.81	14.98	0.29
KPG102B	15.06	14.57	14.22	13.42	0.49	1.64	14.41	0.29
KPG103A	16.27	15.48	14.99	14.18	0.79	2.10	15.66	0.61
KPG103B	15.12	14.47	14.11	13.42	0.64	1.69	14.62	0.47
KPG108A	15.17	13.95	13.49	12.55	1.22	2.62	14.31	1.01
KPG108B	14.35	13.17	12.67	11.76	1.17	2.59	13.45	0.95
KPG112A	13.24	12.41	11.85	11.19	0.83	2.05	12.33	0.60	CP
KPG112B	13.47	12.63	12.05	11.35	0.84	2.12	12.85	0.68	CP
KPG112s	12.55	11.78	11.20	10.52	0.77	2.03
KPG125A	13.65	12.88	12.21	11.39	0.77	2.26	13.16	0.63
KPG125B	12.92	12.15	11.56	10.81	0.77	2.11	12.42	0.63
KPG136A	14.98	14.29	13.72	13.31	0.69	1.67	14.73	0.58
KPG136B	15.21	14.34	13.73	13.24	0.87	1.97	14.87	0.75
KPG141A	15.21	13.96	13.40	12.65	1.25	2.56	14.49	1.06
KPG141B	14.97	13.92	13.50	12.67	1.05	2.30	14.62	0.95
KPG150A	14.24	13.26	12.46	11.90	0.98	2.34	13.84	0.86	BS
KPG150B	13.28	12.23	11.57	10.85	1.05	2.43	12.80	0.95	BS
KPG151A	15.40	14.17	13.42	12.77	1.22	2.63	14.32	0.93	CP
KPG151B	15.07	13.87	13.13	12.53	1.20	2.53	14.60	1.05	CP
KPG151s	14.67	13.33	12.55	11.94	1.34	2.73
KPG156A	13.02	12.60	12.11	11.71	0.42	1.31	12.61	0.30	CP
KPG156B	14.54	13.93	13.42	12.95	0.61	1.59	14.26	0.52	CP
KPG159A	16.39	15.89	15.39	15.45	0.50	0.94	15.90	0.36
KPG159B	14.61	13.88	13.34	12.95	0.73	1.66	14.16	0.60
KPG160A	13.98	13.09	12.58	11.74	0.89	2.23	13.80	0.83
KPG160B	15.44	14.57	14.07	13.23	0.87	2.22	14.83	0.71
KPG168A	12.80	11.82	11.21	10.68	0.99	2.12	12.29	0.86	CP
KPG168B	15.16	14.23	13.73	13.25	0.93	1.91	14.49	0.77	CP
KPG195A	13.24	12.38	11.81	11.20	0.87	2.04	12.87	0.77
KPG195B	14.42	13.71	13.28	12.78	0.71	1.64	13.88	0.58
KPG211A	13.68	12.69	12.01	11.43	0.99	2.25	13.45	0.91
KPG211B	15.75	14.70	13.99	13.39	1.05	2.36	15.05	0.87
KPG216A									BS
KPG216B	13.28	12.70	12.37	11.67	0.58	1.62	12.88	0.36	BS
KPG249A	12.45	12.10	11.67	11.42	0.35	1.03	12.26	0.30	CP
KPG249B	12.93	12.50	12.01	11.68	0.43	1.24	12.51	0.32	CP
KPG249s	11.89	11.49	11.03	10.75	0.40	1.14
KPG295A	13.45	12.59	11.97	11.42	0.86	2.03	13.23	0.79	CP
KPG295B	13.30	12.50	11.92	11.34	0.80	1.97	12.86	0.69	CP
KPG302A	11.23	10.67	10.28	9.64	0.56	1.59	11.03	0.51
KPG302B	14.05	13.57	13.18	12.56	0.46	1.47	13.90	0.42
KPG313A	13.17	12.52	12.10	11.87	0.64	1.30	13.09	0.62
KPG313B	13.26	12.17	11.39	10.68	1.09	2.57	12.97	1.02
KPG332A	12.19	11.35	10.97	10.14	0.84	2.05	11.91	0.77
KPG332B	12.71	11.61	11.11	10.09	1.10	2.62	12.04	0.94
KPG347A	12.13	11.34	10.85	10.26	0.79	1.87	11.97	0.74	CP
KPG347B	12.11	11.19	10.61	9.96	0.92	2.15	11.80	0.83	CP
KPG347s	11.40	10.52	9.96	9.39	0.89	2.02
KPG389A	13.69	12.99	12.37	12.15	0.71	1.55	13.40	0.61	CP

 

Table 3: continued.

KPG	B	V	R	I	B-V	B-I	$B_{\rm T}^{0}$	$(B-V)_{\rm T}^{0}$
KPG389B	13.67	12.83	12.23	11.62	0.84	2.06	13.47	0.76	CP
KPG396A	14.78	14.26	13.84	13.17	0.52	1.61	14.18	0.35
KPG396B	14.06	13.65	13.28	12.48	0.41	1.57	13.99	0.36
KPG404A	13.84	13.12	12.60	12.08	0.72	1.76	13.61	0.65	CP
KPG404B	12.57	11.81	11.26	10.63	0.76	1.93	12.33	0.68	CP
KPG426A	14.77	13.78	13.14	12.57	0.99	2.20	14.46	0.88
KPG426B	15.01	14.11	13.50	13.02	0.90	1.99	14.84	0.82
KPG440A	13.22	12.94	12.63	12.81	0.28	0.41	13.01	0.22
KPG440B	12.42	11.34	10.87	9.90	1.08	2.52	11.79	0.92
KPG455A	13.99	13.11	12.61	11.80	0.88	2.19	13.66	0.78
KPG455B	13.10	12.47	12.02	11.17	0.63	1.93	12.61	0.50

CP = Pair apparently in Contact.
BS = Bright Star nearby in the Field.

It is important to note that a high fraction of the RC3 data available for our pairs comes from Zwicky photographic magnitudes that were transformed to the $B_{\rm T}$ system ((p) in Table 2). The possibility of systematic errors in the Zwicky magnitudes has been discussed by numerous authors (cf. Haynes & Giovanelli 1984). Although these and other authors present recursion relations to convert Zwicky magnitudes to those of other systems, most notably to the photographic magnitudes in the Holmberg system (1958), or to the $B_{\rm T}$ system (de Vaucouleurs et al. 1991), it has been shown that these recursion relations are probably unsatisfactory for magnitudes fainter than 14.0. For this reason, in Fig. 2 we take into account only the total (asymptotic) magnitudes in RC3 derived by extrapolation either from photoelectric aperture-magnitude data ((a) in Table 2) or from surface photometry with photoelectric zero point ((s) in Table 2). In relation to this comparison, Reshetnikov (1993) reports that the total magnitudes for interacting galaxies in RC3 obtained by means of photographic photometry are ${\sim} 0.2{-}0.3$ brighter compared to magnitudes from surface photometry with photoelectric zero point or by extrapolating the photoelectric data.

Figure 3: Comparison between our CCD data and available CCD data from various authors for (S+S) galaxies.

The next step involves comparison of our CCD magnitudes with other CCD measures in the four colour bands. Figure 3 shows the comparison with filled symbols denoting CCD measurements in the Cousins system, primarily from Han (1992); Reshetnikov (1993); and Laurikainen et al. (1998). Open symbols denote CCD measurements in the Johnson system, mainly Godwin et al. (1977); Doroshenko & Terebizh (1979); de Vaucouleurs & Longo (1988) and Márquez & Moles (1996). Metcalfe et al. (1998) reports B and V band photometry in the Landolt system while R and I are in the Cousins system. Giovanelli et al. (1997) report I-band data from a combination of sources. No attempt has been made to transform from any of the above photometric systems to Cousins system.

Notice that in Fig. 3 there seems to be no clear systematic tendency between the compared data, in spite of the small number of galaxies in common. The sigma values obtained through a comparison (only) in the Cousins systems are 0.25, 0.25, 0.20 and 0.30 in B, V, R and I respectively. However, it is fair to mention that this is not a straightforward comparison, since we are also comparing both intrinsic and extrinsic differences involved in each photometric system as well as differences in the reduction procedures, that are more easily detected at fainter magnitudes.

Finally, for most paired galaxies, more than one long exposure per filter is available. Thus we evaluate in addition, the internal accuracy of our photometry by comparing the total magnitudes derived from the individual exposures. We find rms differences between individual measurements of $\delta(B) \sim 0.06$, $\delta(V) \sim 0.06$, $\delta(R) \sim 0.05$ and $\delta(I) \sim 0.05$. Additionally, by estimating total magnitudes for all galaxies before and after sky subtraction, typical values $\delta(B) \sim 0.06$, $\delta(V) \sim 0.07$, $\delta(R) \sim 0.06$ and $\delta(I) \sim 0.07$ are obtained. In the Appendix, we report additional estimations of magnitudes at three concentric circular apertures for all the paired galaxies in this study.