Relationships among the three sets of equatorial standard stars:  McDonald,
Landolt, and SAAO.  Units are magnitudes.

Simple linear relationships (see Figures 1-5) of the form

  Source_1 = a*(Source_2) + b

as reported by the DataGraph plotting routine.

No rejections.

V:
V_SAAO     = 1.0015*V_Landolt - 0.0085
V_McDonald = 0.9996*V_Landolt + 0.0048
V_McDonald = 0.9976*V_SAAO    + 0.0177

B-V:
(B-V)_SAAO     = 1.0028*(B-V)_Landolt + 0.0026
(B-V)_McDonald = 1.0003*(B-V)_Landolt - 0.0011
(B-V)_McDonald = 0.9983*(B-V)_SAAO    - 0.0038

U-B:
(U-B)_SAAO     = 0.9779*(U-B)_Landolt + 0.0132
(U-B)_McDonald = 0.9930*(U-B)_Landolt - 0.0008
(U-B)_McDonald = 0.9818*(U-B)_SAAO    + 0.0136

V-R:
(V-R)_SAAO     = 0.9896*(V-R)_Landolt - 0.0007
(V-R)_McDonald = 0.9887*(V-R)_Landolt + 0.0083
(V-R)_McDonald = 0.9982*(V-R)_SAAO    + 0.0094

V-I:
(V-I)_SAAO     = 0.9926*(V-I)_Landolt + 0.0015
(V-I)_McDonald = 0.9994*(V-I)_Landolt + 0.0019
(V-I)_McDonald = 1.0067*(V-I)_SAAO    + 0.0007

*****************************************************************

Results of a triangular comparison of standard deviations among McDonald,
Landolt, and SAAO standard star lists.  Units are magnitudes.

No rejections.

N.B.  "Sigma1" = standard deviation
      "SigmaN" = mean error in the mean

Mean differences and sigmas between McDonald and Landolt: 
       <delta>  Sigma1   SigmaN   N
V:     0.0007   0.0089   0.0008  110
B-V:  -0.0009   0.0077   0.0007  110
U-B:  -0.0045   0.0211   0.0020  110
V-R:   0.0037   0.0082   0.0008  110
V-I:   0.0014   0.0092   0.0009  110

Mean differences and sigmas between Landolt and SAAO: 
       <delta>  Sigma1   SigmaN   N
V:     0.0073   0.0058   0.0006   90
B-V:   0.0083   0.0069   0.0007   90
U-B:   0.0165   0.0167   0.0018   90
V-R:   0.0055   0.0046   0.0005   90
V-I:   0.0066   0.0056   0.0006   90

Mean differences and sigmas between McDonald and SAAO: 
       <delta>  Sigma1   SigmaN   N
V:    -0.0041   0.0086   0.0009   92
B-V:  -0.0050   0.0087   0.0009   92
U-B:  -0.0035   0.0205   0.0021   92
V-R:   0.0086   0.0062   0.0007   92
V-I:   0.0058   0.0081   0.0008   92

N.B.  (Sigma_source1)^2 = (sigma_12^2 - sigma_23^2 + sigma_31^2)/2
      (Sigma_source2)^2 = (sigma_12^2 - sigma_31^2 + sigma_23^2)/2
      (Sigma_source3)^2 = (sigma_23^2 - sigma_12^2 + sigma_31^2)/2

      McDonald photometry (source 1)
      Landolt standard stars (source 2)
      SAAO standard stars (source 3)

V:
The input standard deviations among sources are 
sigma_12 = 0.0089
sigma_23 = 0.0058
sigma_31 = 0.0086
The calculated standard deviations of the individual sources are 
sigma_1 = 0.0077
sigma_2 = 0.0044
sigma_3 = 0.0038

B-V:
The input standard deviations among sources are 
sigma_12 = 0.0077
sigma_23 = 0.0069
sigma_31 = 0.0087
The calculated standard deviations of the individual sources are 
sigma_1 = 0.0066
sigma_2 = 0.0040
sigma_3 = 0.0057

U-B:
The input standard deviations among sources are 
sigma_12 = 0.0211
sigma_23 = 0.0167
sigma_31 = 0.0205
The calculated standard deviations of the individual sources are 
sigma_1 = 0.0171
sigma_2 = 0.0123
sigma_3 = 0.0113

V-R:
The input standard deviations among sources are 
sigma_12 = 0.0082
sigma_23 = 0.0046
sigma_31 = 0.0062
The calculated standard deviations of the individual sources are 
sigma_1 = 0.0065
sigma_2 = 0.0050
sigma_3 = 0.0020 (sigma_3^2 is negative; assumed positive for square root)

V-I:
The input standard deviations among sources are 
sigma_12 = 0.0092
sigma_23 = 0.0056
sigma_31 = 0.0081
The calculated standard deviations of the individual sources are 
sigma_1 = 0.0077
sigma_2 = 0.0050
sigma_3 = 0.0025