K2IV

JM53, Houk88, Keenan89 do not list any subgiant (class IV) standards
later than K1. Actual K2IV stars are fairly rare, roughly defining the
red edge of the subgiant branch, and the literature seems to have many
main sequence stars misclassified as subgiants (suggesting that the
differences in the spectra may be fairly subtle). Landi Dessy & Keenan
(1966) say "we are in general agreement with Van de Kamp and Mendoza,
V. and have found no subgiants of type later than K2.". They list eps
Ret (HR 1355 = HD 27442) as the only K2IVa in their survey.

MK43 standard: alf Ser - K2III-IV
     	       bet Oph - K2III-IV 

Gray06 lists 7 K2IV stars (and 5 K2IV-V stars), and Gray03 lists 1
K2IV star (and 2 K2IV-V stars)


eps Ret (K2IVa), mu2 Pav (K2IVCNII), HR 3913 (HD 85655, K2IVCNIV-Vp)
and HR 4580 (HD 104055; K2IV) are the brightest K2IVs in the Bright
Star Catalog 5th ed. (Hoffliet91). 

From Hipparcos (1997) stars classified as "K2IV" within 75 pc:
<B-V>(K2IV) = 1.021 (+-0.059 sem, +-0.122 stdev; N=6)
<Mv>(K2IV)  = 4.734 (+-1.111 sem, +-2.251 stdev; N=6)
Four of these six are actually more than one magnitude above the main sequence,
among these:
<B-V>(K2IV,delta(Mv)<-1) = 1.074 (+-0.046 sem, +-0.070 stdev; N=4)
<Mv>(K2IV,delta(Mv)<-1)  = 3.703 (+-0.836 sem, +-1.281 stdev; N=4)

Among Gray03 and Gray06 stars within 40 pc:
B-V(K2IV) = 0.917 (+- 0.018 sem, +- 0.041 stdev; N=7)
V-I(K2IV) = 0.952 (+- 0.034 sem, +- 0.065 stdev; N=7)

The brightest K2IV in the Hipparcos catalog within 75 pc is HD 27442 (eps Ret).

(B-V)(K2IV) = 0.956 ; exemplar HD 194433A = HR 7808A (1 of 2 SIMBAD K2IV d<75pc)
(B-V)(K2IV) = 1.020 ; exemplar HD 10486 = HR 495 (1 of 2 SIMBAD K2IV d<75pc)
(B-V)(K2IV) = 1.04  ; poly fit to FGK subgiant standards & exemplars
(B-V)(K2IV) = 1.044 ; exemplar HD 103403 = HIP 58024 
(B-V)(K2IV) = 1.078 ; exemplar eps Ret = HD 27442 = HIP 19921
=> adopt (B-V)(K2IV) = 1.04 [updated 2/3/2022]

Mv(K2IV) = 1.80  ; fit to SIMBAD d<75pc IIIs for (B-V)=1.04 
Mv(K2IV) =  2.60 ; exemplar HD 10486 = HR 495 (1 of 2 SIMBAD K2IV d<75pc)
Mv(K2IV) =  2.82 ; quadratic trend G0IV-K2IV standards+exemplars (for K2IV)
Mv(K2IV) =  3.14 ; exemplar eps Ret = HD 27442 = HIP 19921
Mv(K2IV) =  3.39 ; exemplar HD 194433A = HR 7808A (1 of 2 SIMBAD K2IV d<75pc)
Mv(K2IV) =  3.75 ; exemplar HD 103403 = HIP 58024
=> adopt Mv(K2IV) = 1.80 [updated 1/29/2022]

# Standards

alf Ser
  K2III-IV: MK43

bet Oph
  K2III-IV: MK43

# Exemplars

eps Ret = HD 27442 = HR 1355 = HIP 19921
  K2IVa: BSC(Hoffleit91),LandiDessyKeenan66
 *K2IV: Houk75(Garrison+Hagen type) 
  K2III: Gray06
  K1/2III: Houk75

  Good agreement on subtype (K2) but some dispersion on luminosity
  class, a although its Mv (3.1) is in family with other K2IVs and
  clearly not giant (Mv ~ 1.2; K2III standards have Mv ~ 0.0, 0.8,
  1.0, 1.0). DA white dwarf companion V=13 at sep=11.4".  V=4.44(HIP),
  B-V=1.078+-0.005(HIP), B-V=1.08(vanAltena95), V-I=1.05+-0.02(HIP),
  U-B=1.07(vanAltena95).  plx=54.83+-0.15mas(HIP2) => Mv=3.135.

HD 194433A = HR 7808A = HIP 100852 A = GJ 9695A 
  K1IV: Adams35,Houk82
  K2IV/V: Evans57
 *K2IV: Corbally80,Corbally84

  1.4" binary (** R 321) with HR 7808B K1V (Corbally80,Corbally84).
  Izmailov19 reports orbit: P=177.3829yr, a=1.04745". 
  Hipparcos splits binary into Hp(A)=6.637, Hp(B)=8.153.
  Tycho-2: Vt(A)=6.584, Vt(B)=8.088, del(Vt)=1.504.
  ASCC: V(A)=6.357+-0.008, V(B)=7.998+-0.013, B-V(A)=0.956. 
  plx(A)=25.4704+-0.0487mas(GaiaEDR3) => Mv(A)=3.387+-0.009.

HD 10486 = HR 495 = HIP 8044
  K0: HD
  K1III-IV: Janes71
  K1IV: Fehrenbach61
 *K2IV: Adams35,Deeming60,Spinrad69

  Star appears to be a very metal-rich subgiant. 
  V=6.33(HIP), B-V=1.020+-0.008(HIP), V-I=0.99+-0.09(HIP).
  plx=17.9628+-0.0961mas(GaiaDR2) => Mv=2.60.  PASTEL - two pairs of
  values given, from Luck95 & Luck17 - just adopt latter:
  Teff=4661K(Luck17), logg=3.08(Luck17), [Fe/H]=+0.36(Luck17).

HD 103403 = HIP 58024
 *K2IV: Houk75

  V=8.08(HIP), B-V=1.044+-0.002(HIP), V-I=1.10+-0.00(HIP).
  plx=13.6213+-0.0332mas(GaiaDR2) => Mv=3.75. 

# Other Stars