**The Kracht Group Pair
C/2002 S7 = 2008 N4**

The orbital linkage was published on MPEC 2008-P60.

Light curves:

C3+C2, phase angles 114 to 17 degree, M. Knight's
measurements are displayed with black dots and my measurements
with red dots.

C/2008 N4 was observed for less than four hours when it was
behind the sun with phase angles of 34 to 19 degree

resulting in very small (<0.1mag) corrections.

My measurements of C/2002 S7 (black dots) and C/2008 N4 (blue
dots) corrected for phase angle and distance.

C/2008 N4 was observed only when it entered the C2 field before
perihelion but not when it left C2 after perihelion.

Karl Battams (IAUC 8985) described the brightness of C/2008 N4 as
"mag about 7.5".

Conditions were favourable at it's previous perihelion 1996
December 6.0. The three 1996 December comets

have perihelion times of December 06.17, 06.28, 06.33. Their
light curves look different from that of C/2002 S7:

C/1999 X3 (blue), X4 (red), X5 (green).

Only 7 from the 33 Kracht comets were found at large phase
angles indicating that there is no significant brightness

enhancement due to forward scattering for the Kracht comets.

Without the corrections for phase angles the light curves look
more similar to that of C/2002 S7:

C/1999 X3 (blue), X4 (red), X5 (green), C/2002 S7 (black).

Any linkage between C/2002 S7 = 2008 N4 and one of the 1996
December comets needs non-gravitational forces.

Actually all three 1996 December comets can be linked with the
pair C/2002 S7 = 2008 N4, but the nongravitational

parameter A1 remains indeterminate and was set to zero in my
calculations. The linkages with C/1996 X3/X4/X5 have

A2 = +0.0027/+0.0050/+0.0058. For these calculations I have used
all observations of C/2002 S7 and 2008 N4 without

the last three observations of C/2008 N4. The mean residual of
the 2002 and 2008 observations is then 25.583/26.375/

27.980" (all C3 observations of C/2002 S7 included).

The best solution is with C/1996 X3 = 2002 S7 = 2008 N4 (and this
solution has the smallest A2).

Light curve of this linkage:

C/1996 X3 (blue), 2002 S7 (black), 2008 N4 (red)

C/2008 N4 lines up nicely with 1996 X3, this linkage looks
good.

Furthermore the residuals of my linkage of C/1996 X3 = 2002 S7 =
2008 N4 look better than those of

C/2002 S7 = 2008 N4 on MPEC 2008-P60 without the need to exclude
so much observations.