Added VARSTAR.bas — a BASIC program for variable-star magnitude using the standard two-comparison-star method (AAVSO / Henden style).

What it does
You enter:

1. Catalog magnitudes of comparison stars 1 and 2 (M1, M2)

2. Measured brightness (counts, flux, ADU, etc.) for comp1, comp2, and the variable (B1, B2, V1)

When your comps match the catalog (D2 ≈ D1), the correction term is small. Instrumental magnitudes use log₁₀ via LOG(x)*K with K = 1/LOG(10).

Example
Input       Value
M1, M2        10.5, 11.0
B1, B2, V1   1000, 800, 950
→ V ≈ 10.024

The program rejects non-positive brightness and warns if the two comps are too close in measurement (D2 ≈ 0), which would make the correction unreliable.

05 REM--VARIABLE STAR MAGNITUDE FROM TWO COMP STARS
10 REM--AAVSO / HENDEN TWO-COMPARISON METHOD
15 REM--V = M1 + D3 - (M1-M2)*(D2-D1)/D2
20 REM--D2, D3 USE LOG10 VIA LOG()*K, K=1/LOG(10)
30 PRINT "VARIABLE STAR PHOTOMETRY"
40 PRINT "TWO COMPARISON STARS"
50 PRINT
60 PRINT "CATALOG MAGNITUDES OF COMP STARS"
70 INPUT M1,M2
80 PRINT "MEASURED BRIGHTNESS (COUNTS/FLUX)"
90 PRINT "COMP1, COMP2, VARIABLE"
100 INPUT B1,B2,V1
110 IF B1<=0 OR B2<=0 OR V1<=0 THEN 500
120 LET K=1/LOG(10)
130 LET D1=M1-M2
140 LET D2=-2.5*LOG(B1/B2)*K
150 LET D3=-2.5*LOG(V1/B1)*K
160 IF ABS(D2)<1E-12 THEN 510
170 LET V=M1+D3-(M1-M2)*(D2-D1)/D2
180 PRINT
190 PRINT "INSTR. DIFF C1-C2 (D2)=";D2
200 PRINT "KNOWN DIFF       (D1)=";D1
210 PRINT "INSTR. DIFF VAR-C1 (D3)=";D3
220 PRINT
230 PRINT "VARIABLE MAGNITUDE V =";V
240 END
500 PRINT "BRIGHTNESS MUST BE POSITIVE"
510 PRINT "COMP STARS TOO CLOSE - CHECK DATA"
520 END