We present a structural analysis of nearby galaxies in spiral-spiral pairs in optical BVRI bands and compare with the structures of isolated spiral galaxies and galaxies in ongoing mergers .
We use these comparisons to determine how galaxy structure changes during galaxy interactions and mergers .
We analyze light concentration ( C ) , asymmetry ( A ) , and clumpiness ( S ) parameters , and use the projections of CAS parameter space to compare these samples .
We find that the CAS parameters of paired galaxies are correlated with the projected separations of the pair .
For the widest and closest pairs , the CAS parameters tend to be similar to those of isolated and ongoing major mergers ( ULIRGs ) , respectively .
Our results imply that galaxy CAS morphology is a robust quantity that only changes significantly during a strong interaction or major merger .
The typical time-scale for this change in our paired sample , based on dynamical friction arguments , is short , \tau \approx 0.1 - 0.5 Gyr .
We find average enhancement factors for the spiral pair asymmetries and clumpiness values of \sim 2.2 and 1.5 .
The S parameter , which is related to star formation activity , has a moderate level of enhancement suggesting that this activity in modern spirals depends more on internal processes than on external conditions .

We furthermore test the statistical criterion for picking up interacting galaxies in an automated way by using the A - S projection plane .
The diversity of our spiral pair sample in the CAS space suggests that structural/SF/morphological properties of interacting galaxies change abruptly only when the interaction becomes very strong and that the criteria for finding galaxies involved in major mergers from Conselice ( 2003 ) is effective .