of ${\rm E}({\rm B}-{\rm V})_{\rm map}$ to ${\rm E}({\rm B}-{\rm
V})$ from Strömgren photometry for 150 A-type stars. A least-squares optimisation algorithm was used to find an initial set of parameters for the chain and the Markov chains were calculated using 64 walkers and 256 steps following a burn-in run of 128 steps. An example of the output from the program used to implement our method is shown in Fig. \[fitmag\].


    Read 17 lines from EPIC201161715.phot

     Calculating least-squares solution...
      g_1     = 15.50
      T_eff,1 = 4931 K
      g_2     = 17.25
      T_eff,2 = 5264 K
      E(B-V)  =   0.00
      chi-squared = 3.44
      Ndf = 12
      sigma_ext = 0.05

     Starting emcee chain of 384 steps with 64 walkers

     Median acceptance fraction = 0.487
     Best log(likelihood) = 42.52 in walker 12 at step 155

     Parameter median values, standard deviations and best-fit values.
      g_1     = 15.362 +/-  0.102   [ 15.460 ]
      T_eff,1 =   5031 +/-   84 K     [ 4958 ]
      g_2     = 17.148 +/-  0.207   [ 17.231 ]
      T_eff,2 =   5366 +/-  120 K     [ 5282 ]
      E(B-V)  =  0.034 +/-  0.025   [  0.011 ]
      sig_ext =  0.027 +/-  0.016   [  0.019 ]
      chi-squared = 9.90

    type band value_obs error   source  value_fit value_A  value_B   z
    ---- 